ENGINEERING

UDC 62l.43.052

TECHNICAL IMPLEMENTATION OF CHANGING THE COMPRESSION RATE OF A SMALL ENGINE POWERED ON NATURAL GAS

F.I. Abramchuk, Professor, Doctor of Technical Sciences, A.N. Kabanov, Associate Professor, Ph.D.,

A.P. Kuzmenko, PhD student, KhNADU

Annotation. The results of the technical implementation of the change in the compression ratio on the MeMZ-307 engine, which is re-equipped for operation on natural gas.

Keywords: compression ratio, car engine, natural gas.

TECHNICAL IMPLEMENTATION OF CHANGING THE STAGE OF CLOSING A SMALL CAR ENGINE,

WHAT WORK ON NATURAL GAS

F.I. Abramchuk, Professor, Doctor of Technical Sciences, O.M. Kabanov, Associate Professor, Ph.D.,

A.P. Kuzmenko, PhD student, KhNADU

Abstract. The results of the technical implementation of the change of the compression stage of the MeMZ-307 engine, re-domination for the operation on natural gas are given.

Key words: squeezing stage, car engine, natural gas.

TECHNICAL REALIZATION OF COMPRESSION RATIO VARIATION OF SMALL-CAPACITY AUTOMOTIVE NATURAL GAS POWERED ENGINE

F. Abramchuk, Professor, Doctor of Technical Science, A. Kabanov, Associate Professor, Doctor of Technical Science, A. Kuzmenko, postgraduate, KhNAHU

abstract. The results of technical realization of compression ratio variation of MeMZ-3Q7 engine converted for natural gas running are given.

Key words: compression ratio, automotive engine, natural gas.

Introduction

The creation and successful operation of purely gas engines that run on natural gas depend on the correct choice of the main parameters of the working process that determine their technical, economic and environmental characteristics. First of all, this concerns the choice of compression ratio.

Natural gas, having a high octane number (110-130), allows you to increase the compression ratio. Maximum degree value

compression, excluding detonation, can be chosen in the first approximation by calculation. However, it is possible to verify and refine the calculated data only experimentally.

Publication analysis

In progress while translating gasoline engine(Vh = 1 l) of the VW POLO car for natural gas, the shape of the firing surface of the piston is simplified. Reducing the volume of the compression chamber led to an increase in the compression ratio from 10.7 to 13.5.

On the D21A engine, the piston was reworked to reduce the compression ratio from 16.5 to 9.5. The hemispherical combustion chamber for diesel has been modified for the working process of a gas engine with spark ignition.

When converting the YaMZ-236 diesel engine into a gas engine, the compression ratio was also reduced from 16.2 to 12 due to the reworking of the piston.

Purpose and problem statement

The aim of the work is to develop the design of the combustion chamber parts of the MeMZ-307 engine, which makes it possible to provide a compression ratio of e = 12 and e = 14 for experimental studies.

Choosing an Approach to Changing the Compression Ratio

For a small petrol engine convertible to gas, changing the compression ratio means increasing it compared to the base internal combustion engine. There are several ways to accomplish this task.

In the ideal case, it is desirable to install a system for changing the compression ratio on the engine, which allows this task to be performed in real time, including without interrupting engine operation. However, such systems are very expensive and complex in design and operation, require significant changes in the design, and are also an element of unreliability of the engine.

You can also change the compression ratio by increasing the number or thickness of the gaskets between the head and the cylinder block. This method is cheap, but it increases the likelihood of gaskets burning out in case of violation normal process fuel combustion. In addition, this method of regulating the compression ratio is characterized by low accuracy, since the value of e will depend on the tightening force of the nuts on the head studs and the quality of the gaskets. Most often, this method is used to lower the compression ratio.

The use of linings on pistons is technically difficult, since there is a problem of reliable fastening of a relatively thin lining (about 1 mm) to the piston and reliable operation this mount under combustion chamber conditions.

The best option is the manufacture of sets of pistons, each of which provides a given degree of compression. This method requires partial disassembly of the engine to change the compression ratio, however, it provides a sufficiently high accuracy of the value of e in the experiment and the reliability of the engine with a changed compression ratio (the strength and reliability of the engine structural elements are not reduced). In addition, this method is relatively cheap.

Research results

The essence of the problem was to use positive traits natural gas (high octane number) and features of mixture formation, to compensate for the loss of power when the engine is running on this fuel. To accomplish the task, it was decided to change the compression ratio.

According to the experimental plan, the compression ratio should change from e = 9.8 (standard equipment) to e = 14. It is advisable to choose an intermediate value of the compression ratio e = 12 (as the arithmetic average of the extreme values ​​of e). If necessary, it is possible to manufacture sets of pistons that provide other intermediate values ​​of the compression ratio.

For the technical implementation of the indicated compression ratios, calculations, design developments and experimentally verified volumes of compression chambers by the pouring method were performed. The spill results are shown in tables 1 and 2.

Table 1 Results of flushing the combustion chamber in the cylinder head

1 cyl. 2 cyl. 3 cyl. 4 cyl.

22,78 22,81 22,79 22,79

Table 2 The results of the combustion chamber flushing in the pistons (the piston is installed in the cylinder)

1 cyl. 2 cyl. 3 cyl. 4 cyl.

9,7 9,68 9,71 9,69

The thickness of the gasket in the compressed state is 1 mm. Piston sinking relative to the plane of the cylinder block is 0.5 mm, which was determined using measurements.

Accordingly, the volume of the combustion chamber Vc will consist of the volume in the cylinder head Ug, the volume in the piston Vn and the volume of the gap between the piston and the cylinder head (piston sinking relative to the plane of the cylinder block + gasket thickness) Ush = 6.6 cm3.

Vc = 22.79 + 9.7 + 4.4 = 36.89 (cm3).

It was decided to change the compression ratio by changing the volume of the combustion chamber by changing the geometry of the piston head, since this method allows you to implement all options for the compression ratio, and at the same time it is possible to return to the standard configuration.

On fig. Figure 1 shows the serial configuration of the combustion chamber parts with piston volumes Yn = 7.5 cm3.

Rice. one. Standard equipment combustion chamber parts Yc = 36.9 cm3 (e = 9.8)

To obtain a compression ratio e = 12, it is sufficient to complete the combustion chamber with a piston with a flat bottom, in which two small samples are made with a total volume

0.1 cm3, preventing intake and exhaust valves from meeting the piston during

overlap. In this case, the volume of the compression chamber is

Vc = 36.9 - 7.4 = 29.5 (cm3).

In this case, the gap between the piston and the cylinder head remains 8 = 1.5 mm. The design of the combustion chamber, providing є = 12, is shown in fig. 2.

Rice. 2. Complete set of parts of the combustion chamber of a gas engine to obtain a compression ratio є = 12 (Us = 29.5 m3)

It is accepted to realize the compression ratio є = 14 by increasing the height of the piston with a flat bottom by H = 1 mm. In this case, the piston also has two selections for valves with a total volume of 0.2 cm3. The volume of the compression chamber is reduced by

DU \u003d - I \u003d. 0.1 = 4.42 (cm3).

This configuration of the combustion chamber parts gives the volume

Vc = 29.4 - 4.22 = 25.18 (cm3).

On fig. 3 shows the configuration of the combustion chamber, providing a compression ratio є = 13.9.

The gap between the firing surface of the piston and the cylinder head is 0.5 mm, which is sufficient for the normal operation of the parts.

Rice. 3. Complete set of parts of the combustion chamber of a gas engine with e = 13.9 (Us = 25.18 cm3)

1. Simplification of the geometric shape of the firing surface of the piston (flat head with two small selections) made it possible to increase the compression ratio from 9.8 to 12.

2. Reducing the gap to 5 = 0.5 mm between the cylinder head and the piston at TDC and simplifying the geometric shape of the firing line

piston surface allowed to increase є to 13.9 units.

Literature

1. According to the website: www.empa.ch

2. Bgantsev V.N. gas engine on the base

four-stroke general purpose diesel / V.N. Bgantsev, A.M. Levterov,

B.P. Marakhovsky // Mir tekhniki i tekhnologii. - 2003. - No. 10. - S. 74-75.

3. Zakharchuk V.I. Rozrakhunkovo-experiment-

talne dosl_dzhennya gas engine, re-engineered diesel / V.I. Zakharchuk, O.V. Sitovsky, I.S. Kozachuk // Automobile transport: Sat. scientific tr. -Kharkov: KHNADU. - 2005. - Issue. sixteen. -

4. Bogomolov V.A. Design features

experimental setup for research of gas engine 64 13/14 with spark ignition / V.A. Bogomolov, F.I. Abramchuk, V.M. Manoylo and others // Bulletin of KhNADU: Sat. scientific tr. - Kharkov: KHNADU. -2007. - No. 37. - S. 43-47.

Reviewer: M. A. Podrigalo, Professor, Doctor of Technical Sciences, KhNADU.

1

1 State Scientific Center of the Russian Federation - Federal State Unitary Enterprise "Central Order of the Red Banner of Labor Research Automobile and Automotive Institute (NAMI)"

When converting a diesel engine to a gas engine, supercharging is used to compensate for the decrease in power. To prevent detonation, the geometric compression ratio is reduced, which causes a decrease in the indicator efficiency. Differences between the geometric and actual compression ratios are analyzed. Closing the intake valve by the same amount before or after BDC causes the same reduction in the actual compression ratio compared to the geometric compression ratio. A comparison of the parameters of the filling process with a standard and shortened intake phase is given. It is shown that the early closing of the intake valve allows to reduce the actual compression ratio, lowering the knock threshold, while maintaining a high geometric compression ratio and high indicator efficiency. The shortened inlet provides an increase in mechanical efficiency by reducing the pressure of pumping losses.

gas engine

geometric compression ratio

actual compression ratio

valve timing

indicator efficiency

mechanical efficiency

detonation

pumping losses

1. Kamenev V.F. Prospects for improving the toxic performance of diesel engines of vehicles weighing more than 3.5 tons / V.F. Kamenev, A.A. Demidov, P.A. Shcheglov // Proceedings of NAMI: Sat. scientific Art. - M., 2014. - Issue. No. 256. - P. 5–24.

2. Nikitin A.A. Adjustable actuator of the valve for inlet of the working medium into the engine cylinder: Pat. 2476691 the Russian Federation, IPC F01L1/34 / A.A. Nikitin, G.E. Sedykh, G.G. Ter-Mkrtichyan; applicant and patent holder SSC RF FSUE "NAMI", publ. 02/27/2013.

3. Ter-Mkrtichyan G.G. Engine with quantitative throttleless power control // Automotive industry. - 2014. - No. 3. - P. 4-12.

4. Ter-Mkrtichyan G.G. Scientific basis for the creation of engines with a controlled compression ratio: dis. doc. … tech. Sciences. - M., 2004. - 323 p.

5. Ter-Mkrtichyan G.G. Piston control in engines internal combustion. - M. : Metallurgizdat, 2011. - 304 p.

6. Ter-Mkrtichyan G.G. Trends in the development of battery fuel systems for large diesel engines / G.G. Ter-Mkrtichyan, E.E. Starkov // Proceedings of NAMI: Sat. scientific Art. - M., 2013. - Issue. No. 255. - S. 22-47.

Recently, there has been widespread use in trucks and buses find gas engines that are converted from diesels by modifying the cylinder head with the replacement of the nozzle with a spark plug and equipping the engine with equipment for supplying gas to the intake pipeline, or intake channels. To prevent detonation, the compression ratio is lowered, as a rule, by modifying the piston.

A gas engine a priori has less power and worse fuel efficiency compared to the base diesel. The decrease in the power of a gas engine is explained by a decrease in the filling of the cylinders with an air-fuel mixture due to the replacement of part of the air with a gas that has a larger volume compared to liquid fuel. To compensate for the reduction in power, supercharging is used, which requires an additional reduction in the compression ratio. At the same time, the indicator Engine efficiency accompanied by deterioration in fuel efficiency.

As base engine for conversion to gas, a diesel engine of the YaMZ-536 family (6CHN10.5 / 12.8) with a geometric compression ratio was chosen ε \u003d 17.5 and a rated power of 180 kW at a speed of crankshaft 2300 min -1 .

Fig.1. Dependence of the maximum power of a gas engine on the degree of compression (detonation limit).

Figure 1 shows the dependence of the maximum power of a gas engine on the compression ratio (detonation limit). In a converted engine with standard valve timing, the specified rated power of 180 kW without detonation can only be achieved with a significant reduction in the geometric compression ratio from 17.5 to 10, causing a noticeable decrease in indicator efficiency.

Detonation can be avoided without a decrease or with a minimum decrease in the geometric compression ratio, and hence a minimum decrease in the indicator efficiency, by implementing a cycle with early closing of the intake valve. In this cycle, the intake valve closes before the piston reaches BDC. After the intake valve is closed, when the piston moves to the BDC, the gas-air mixture first expands and cools, and only after the piston passes through the BDC and moves to the TDC, it begins to compress. The loss of filling of the cylinders is compensated for by increasing the boost pressure.

The main objectives of the research were to identify the possibility of converting a modern diesel engine into a gas engine with external mixture formation and quantitative control while maintaining high power and fuel efficiency of the base diesel engine. Let's consider some key moments of approaches to the decision of tasks in view.

Geometric and actual compression ratios

The start of the compression process coincides with the closing moment of the intake valve φ a. If this happens at LDC, then the actual compression ratio ε f is equal to the geometric compression ratio ε. With the traditional organization of the working process, the inlet valve closes 20-40 ° after BDC in order to improve filling due to recharging. In a short intake cycle, the intake valve closes to BDC. Therefore, in real engines the actual compression ratio is always less than the geometric compression ratio.

Closing the intake valve by the same amount either before or after BDC causes the same decrease in the actual compression ratio compared to the geometric compression ratio. So, for example, when changing φ a 30° before or after BDC, the actual compression ratio is reduced by approximately 5%.

Changing the parameters of the working body during filling

During the research, the standard exhaust phases were retained, and the intake phases were changed by varying the closing angle of the intake valve φ a. In this case, with early closing of the intake valve (before BDC) and maintaining the standard intake duration (Δφ vp= 230°), the intake valve would have to be opened long before TDC, which, due to the large valve overlap, would inevitably lead to an excessive increase in the residual gas ratio and disturbances in the flow of the working process. Therefore, the early closing of the intake valve required a significant reduction in intake duration to 180°.

Figure 2 shows a diagram of charge pressure during filling as a function of the closing angle of the inlet valve to BDC. Pressure at the end of filling p a lower than the pressure in the intake manifold, and the pressure decrease is the greater, the earlier the intake valve closes to BDC.

When the intake valve is closed at TDC, the charge temperature at the end of filling T a slightly higher than the temperature in the inlet pipeline T k. When the intake valve closes earlier, the temperatures approach each other, and when φ a>35...40° PCV charge does not heat up during filling, but cools down.

1 - φ a=0°; 2 - φ a=30°; 3 - φ a=60°.

Fig. 2. Influence of the closing angle of the inlet valve on the change in pressure during the filling process.

Optimization of the intake phase at rated power

Ceteris paribus, boosting or increasing the compression ratio in engines with external mixture formation is limited by the same phenomenon - the occurrence of detonation. Obviously, with the same excess air coefficient and the same ignition timing, the conditions for the onset of detonation correspond to certain pressure values pc and temperature Tc charge at the end of compression, depending on the actual compression ratio .

For the same geometric compression ratio and, consequently, the same compression volume, the ratio pc/ Tc uniquely determines the amount of fresh charge in the cylinder. The ratio of the pressure of the working fluid to its temperature is proportional to the density. Therefore, the actual compression ratio shows how much the density of the working fluid increases during the compression process. The parameters of the working fluid at the end of compression, in addition to the actual degree of compression, are significantly affected by the pressure and temperature of the charge at the end of filling, which are determined by the course of gas exchange processes, primarily the filling process.

Consider engine options with the same geometric compression ratio and the same mean indicator pressure, one of which has a standard intake duration ( Δφ vp=230°), and in the other the inlet is shortened ( Δφ vp\u003d 180 °), the parameters of which are presented in table 1. In the first variant, the inlet valve closes 30 ° after TDC, and in the second variant, the inlet valve closes 30 ° before TDC. Therefore, the actual compression ratio ε f the two variants with late and early closing of the intake valve are the same.

Table 1

Parameters of the working fluid at the end of filling for a standard and shortened inlet

Δφ vp, °	φ a, °	P k, MPa		Pa, MPa		ρ a, kg / m 3

The average indicator pressure at a constant value of the excess air coefficient is proportional to the product of the indicator efficiency and the amount of charge at the end of filling. The indicator efficiency, other things being equal, is determined by the geometric compression ratio, which is the same in the options under consideration. Therefore, the indicator efficiency can also be assumed to be the same.

The amount of charge at the end of filling is determined by the product of the charge density at the inlet and the filling factor ρ kηv. The use of efficient charge air coolers makes it possible to keep the charge temperature in the intake manifold approximately constant, regardless of the degree of pressure increase in the compressor. Therefore, we will assume as a first approximation that the charge density in the intake manifold is directly proportional to the boost pressure.

In the variant with a standard intake duration and inlet valve closing after BDC, the filling ratio is 50% higher than in the variant with a short intake and inlet valve closing to BDC.

With a decrease in the filling ratio, in order to maintain the average indicator pressure at a given level, it is necessary to proportionally, i.e. by the same 50%, increase the boost pressure. In this case, in the variant with early closing of the inlet valve, both the pressure and temperature of the charge at the end of filling will be 12% lower than the corresponding pressure and temperature in the variant with closing of the inlet valve after BDC. Due to the fact that in the variants under consideration the actual compression ratio is the same, the pressure and temperature of the end of compression in the variant with early closing of the intake valve will also be 12% lower than when the intake valve is closed after BDC.

Thus, in an engine with a shortened intake and closing the intake valve to BDC, while maintaining the same average indicator pressure, the likelihood of detonation can be significantly reduced compared to an engine with a standard intake duration and closing the intake valve after BDC.

Table 2 compares the parameters of gas engine options when operating at nominal mode.

table 2

Parameters of gas engine options

option number
Compression ratio ε
Inlet valve opening φ s, ° PCV
Inlet valve closing φ a, ° PCV
Compressor pressure ratio pk
Pumping loss pressure pnp, MPa
Mechanical loss pressure pm, MPa
Filling ratio η v
Indicator efficiency η i
Mechanical efficiency η m
Effective efficiency η e
Compression start pressure p a, MPa
Compression start temperature T a, K

Figure 3 shows the gas exchange diagrams for different intake valve closing angles and the same filling time, while Figure 4 shows the gas exchange diagrams for the same actual compression ratio and different filling times.

In the rated power mode, the closing angle of the inlet valve φ a=30° to BDC actual compression ratio ε f=14.2 and the degree of pressure increase in the compressor π k=2.41. This ensures the minimum level of pumping losses. With an earlier closing of the intake valve due to a decrease in the filling ratio, it is required to significantly increase the boost pressure by 43% (π k=3.44), which is accompanied by a significant increase in pumping loss pressure.

With an early closing of the intake valve, the charge temperature at the beginning of the compression stroke T a, due to its pre-expansion, is 42 K lower compared to an engine with standard intake phases.

Internal cooling of the working fluid, accompanied by the removal of part of the heat from the hottest elements of the combustion chamber, reduces the risk of detonation and glow ignition. The filling factor is reduced by a third. It becomes possible to work without detonation with a compression ratio of 15, against 10 with a standard intake duration.

1 - φ a=0°; 2 - φ a=30°; 3 - φ a=60°.

Rice. 3. Diagrams of gas exchange at different intake valve closing angles.

1-φ a=30°before TDC; 2-φ a\u003d 30 ° behind TDC.

Fig.4. Diagrams of gas exchange at the same actual compression ratio.

The time-section of the intake valves of the engine can be changed by adjusting the height of their rise. One of the possible technical solutions is the intake valve lift control mechanism developed at SSC NAMI. The development of hydraulically driven devices for independent electronic control of the opening and closing of valves, based on the principles industrially implemented in accumulators, has great prospects. fuel systems diesels.

Despite the increase in boost pressure and the higher compression ratio in the short intake engine, due to the early closing of the intake valve and therefore more low pressure the beginning of compression, the average pressure in the cylinder does not increase. Therefore, the friction pressure also does not increase. On the other hand, with a shortened inlet, the pressure of pumping losses decreases significantly (by 21%), which leads to an increase in mechanical efficiency.

The implementation of a higher compression ratio in an engine with a short intake causes an increase in indicated efficiency and, in combination with a slight increase in mechanical efficiency, is accompanied by an increase in effective efficiency by 8%.

Conclusion

The results of the studies carried out indicate that the early closing of the intake valve makes it possible to manipulate the filling ratio and the actual compression ratio in a wide range, reducing the knock threshold without reducing the indicator efficiency. The shortened inlet provides an increase in mechanical efficiency by reducing the pressure of pumping losses.

Reviewers:

Kamenev V.F., Doctor of Technical Sciences, Professor, Leading Expert, State Scientific Center of the Russian Federation FSUE "NAMI", Moscow.

Saikin A.M., Doctor of Technical Sciences, Head of Department, SSC RF FSUE "NAMI", Moscow.

Bibliographic link

Ter-Mkrtichyan G.G. CONVERSION OF A DIESEL INTO A GAS ENGINE WITH A DECREASE IN THE ACTUAL COMPRESSION RATE // Modern Problems of Science and Education. - 2014. - No. 5.;
URL: http://science-education.ru/ru/article/view?id=14894 (date of access: 01.02.2020). We bring to your attention the journals published by the publishing house "Academy of Natural History"

Evgeny Konstantinov

While gasoline and diesel fuel are inexorably becoming more expensive, and all kinds of alternative power plants for vehicles remain terribly far from the people, losing to traditional internal combustion engines in price, autonomy and operating costs, the most real way to save on gas stations is to transfer the car to a “gas diet”. At first glance, this is beneficial: the cost of re-equipping the car will soon pay off due to the difference in fuel prices, especially with regular commercial and passenger traffic. Not without reason, in Moscow and many other cities, a significant proportion of municipal vehicles have long been switched to gas. But here a natural question arises: why, then, the share of gas-balloon vehicles in the traffic flow both in our country and abroad does not exceed a few percent? What is hidden on the back side of the gas cylinder?

Science and life // Illustrations

Warning signs at the gas station are not without reason: each connection of the process gas pipeline is a potential place for leaks of combustible gas.

Liquefied gas cylinders are lighter, cheaper and more diverse in shape than compressed gas, and therefore they are easier to arrange based on the free space in the car and the necessary range.

Pay attention to the difference in the price of liquid and gaseous fuels.

Cylinders with compressed methane in the back of a tilt-covered Gazelle.

The reducer-evaporator in the propane system requires heating. The photo clearly shows the hose connecting the liquid heat exchanger of the gearbox to the engine cooling system.

circuit diagram operation of gas-balloon equipment on a carburetor engine.

Scheme of operation of equipment for liquefied gas without transferring it to the gaseous phase in an internal combustion engine with distributed injection.

Propane-butane is stored and transported in tanks (pictured behind the blue gate). Thanks to this mobility, the gas station can be placed in any convenient place, and if necessary, quickly transferred to another.

At the propane column, not only cars are filled, but also household cylinders.

The column for liquefied gas looks different from gasoline, but the refueling process is similar. The reading of the filled fuel is in liters.

The concept of "gas automotive fuel" includes two mixtures that are completely different in composition: natural gas, in which up to 98% is methane, and propane-butane produced from associated petroleum gas. In addition to unconditional flammability, they also have a common state of aggregation at atmospheric pressure and temperatures comfortable for life. However, when low temperatures the physical properties of these two sets of light hydrocarbons are quite different. Because of this, they require completely different equipment for storage on board and supply to the engine, and in operation, cars with different gas supply systems have several significant differences.

Liquefied gas

The propane-butane mixture is well known to tourists and summer residents: it is it that is filled into household gas cylinders. It also makes up the bulk of the gas that is wasted in the flares of oil producing and processing enterprises. The proportional composition of the fuel propane-butane mixture may vary. The point is not so much in the initial composition of the petroleum gas, but in the temperature properties of the resulting fuel. As a motor fuel, pure butane (C 4 H 10) is good in all respects, except that it passes into a liquid state already at 0.5 ° C at atmospheric pressure. Therefore, less caloric, but more cold-resistant propane (C 2 H 8) with a boiling point of -43 ° C is added to it. The ratio of these gases in the mixture sets the lower temperature limit for the use of fuel, which for the same reason can be "summer" and "winter".

The relatively high boiling point of propane-butane, even in the "winter" version, allows it to be stored in cylinders in the form of a liquid: even under low pressure, it passes into the liquid phase. Hence another name for propane-butane fuel - liquefied gas. It is convenient and economical: the high density of the liquid phase allows you to fit a large amount of fuel in a small volume. The free space above the liquid in the cylinder is occupied by saturated steam. As the gas is consumed, the pressure in the cylinder remains constant until it is empty. Drivers of "propane" cars when refueling should fill the tank to a maximum of 90% to leave room for a vapor cushion inside.

The pressure inside the cylinder primarily depends on the ambient temperature. At negative temperatures, it drops below one atmosphere, but even this is enough to maintain the system's performance. But with warming it grows rapidly. At 20°C, the pressure in the cylinder is already 3-4 atmospheres, and at 50°C it reaches 15-16 atmospheres. For most automotive gas cylinders, these values ​​are close to the limit. And this means that when overheated on a hot afternoon in the southern sun, a dark car with a bottle of liquefied gas on board ... No, it will not explode, like in a Hollywood action movie, but will begin to dump excess propane-butane into the atmosphere through a safety valve designed specifically for such a case . By evening, when it gets cold again, there will be noticeably less fuel in the cylinder, but no one and nothing will be hurt. True, as statistics show, some amateurs additionally save on a safety valve from time to time replenish the chronicle of incidents.

compressed gas

Other principles underlie the operation of gas-balloon equipment for vehicles that consume natural gas as a fuel, commonly referred to as methane in everyday life by its main component. This is the same gas that is supplied through pipes to city apartments. Unlike petroleum gas, methane (CH 4) has a low density (1.6 times lighter than air), and most importantly, a low boiling point. It passes into a liquid state only at –164°C. The presence of a small percentage of impurities of other hydrocarbons in natural gas does not greatly change the properties of pure methane. This means that turning this gas into a liquid for use in a car is incredibly difficult. In the last decade, work has been actively carried out on the creation of so-called cryogenic tanks, which make it possible to store liquefied methane in a car at temperatures of -150 ° C and below and pressures up to 6 atmospheres. Prototypes of transport and gas stations for this fuel option were created. But so far this technology has not received practical distribution.

Therefore, in the vast majority of cases, for use as a motor fuel, methane is simply compressed, bringing the pressure in the cylinder to 200 atmospheres. As a result, the strength and, accordingly, the mass of such a cylinder should be noticeably higher than for propane. Yes, and placed in the same volume of compressed gas is significantly less than liquefied (in terms of moles). And this is a decrease in the autonomy of the car. Another downside is the price. A significantly greater margin of safety incorporated in methane equipment turns out to be that the price of a kit for a car turns out to be almost ten times higher than propane equipment of a similar class.

Methane cylinders come in three sizes, of which passenger car only the smallest ones, with a volume of 33 liters, can be accommodated. But in order to ensure a guaranteed range of three hundred kilometers, five such cylinders are needed, with a total mass of 150 kg. It is clear that in a compact city runabout it makes no sense to constantly carry such a load instead of useful luggage. Therefore, there is a reason to convert to methane only big cars. First of all, trucks and buses.

With all this, methane has two significant advantages over petroleum gas. First, it is even cheaper and is not tied to the price of oil. And secondly, methane equipment is structurally insured against problems with winter operation and allows, if desired, to do without gasoline at all. In the case of propane-butane in our climatic conditions, such a focus will not work. In fact, the car will remain dual-fuel. The reason is the liquefied gas. More precisely, in the fact that in the process of active evaporation, the gas cools sharply. As a result, the temperature in the cylinder drops sharply, and especially in the gas reducer. To prevent the equipment from freezing, the gearbox is heated by embedding a heat exchanger connected to the engine cooling system. But in order for this system to start working, the liquid in the line must first be heated. Therefore, it is recommended to start and warm up the engine at an ambient temperature below 10 ° C strictly on gasoline. And only then, with the output of the motor on operating temperature, switch to gas. However, modern electronic systems switch everything themselves, without the help of a driver, automatically controlling the temperature and preventing the equipment from freezing. True, in order to maintain the correct operation of the electronics in these systems, it is impossible to empty the gas tank to dryness even in hot weather. The starting mode on gas is emergency for such equipment, and the system can only be switched to it forcibly in case of emergency.

Methane equipment has no difficulties with winter start-up. On the contrary, it is even easier to start the engine on this gas in cold weather than on gasoline. The absence of a liquid phase does not require heating of the reducer, which only lowers the pressure in the system from 200 transport atmospheres to one working one.

The wonders of direct injection

The most difficult thing is to switch to gas modern engines with direct injection fuel into the cylinders. The reason is that gas injectors are traditionally located in the intake tract, where mixture formation occurs in all other types of internal combustion engines without direct injection. But the presence of such completely crosses out the possibility of adding gas supply so easily and technologically. Firstly, ideally, gas should also be fed directly into the cylinder, and secondly, and more importantly, liquid fuel serves to cool its own direct injection nozzles. Without it, they very quickly fail from overheating.

There are solutions to this problem, and at least two. The first turns the engine into a dual-fuel. It was invented a long time ago, even before the advent of direct injection on gasoline engines, and was proposed to adapt diesel engines to work on methane. The gas does not ignite from compression, and therefore the "carbonated diesel" starts up on diesel fuel and continues to work on it in the mode idling and minimum load. And then gas comes into play. It is due to its supply that the speed of rotation of the crankshaft is regulated in the mode of medium and high revolutions. For this injection pump ( fuel pump high pressure) limit the supply of liquid fuel to 25-30% of the nominal value. Methane enters the engine through its own line, bypassing the injection pump. There are no problems with its lubrication due to a decrease in the supply of diesel fuel at high speeds. The diesel injectors continue to be cooled by the fuel passing through them. True, the thermal load on them in the high-speed mode still remains increased.

A similar power scheme began to be used for gasoline engines with direct injection. Moreover, it works with both methane and propane-butane equipment. But in the latter case, an alternative solution that has appeared quite recently is considered more promising. It all started with the idea to abandon the traditional evaporator gearbox and supply propane-butane to the engine under pressure in the liquid phase. The next steps were the rejection of gas injectors and the supply of liquefied gas through standard gasoline injectors. An electronic matching module was added to the circuit, connecting a gas or gasoline line according to the situation. Wherein new system lost the traditional problems with a cold start on gas: no evaporation - no cooling. True, the cost of equipment for engines with direct injection in both cases is such that it pays off only with very high mileage.

By the way, economic feasibility limits the use of gas-balloon equipment in diesel engines. It is for reasons of benefit for compression-ignition engines that only methane equipment is used, and only heavy equipment engines equipped with traditional high-pressure fuel pumps are suitable in terms of characteristics. The fact is that the transfer of small economical passenger engines from diesel to gas does not pay for itself, and the development and technical implementation gas balloon equipment for the latest engines with a common fuel rail (common rail) are currently considered economically unjustified.

True, there is another, alternative way to transfer diesel to gas - by completely converting it into a gas engine with spark ignition. In such a motor, the compression ratio decreases to 10-11 units, candles and high-voltage electrics appear, and it says goodbye forever to diesel fuel. But it begins to painlessly consume gasoline.

Working conditions

The old Soviet instructions for converting gasoline cars to gas required grinding cylinder heads (cylinder heads) to raise the compression ratio. This is understandable: the object of gasification in them was the power units of commercial vehicles that ran on gasoline with an octane rating of 76 and below. Methane has an octane rating of 117, while propane-butane mixtures have about a hundred. Thus, both gaseous fuels are significantly less prone to detonation than gasoline and allow the compression ratio of the engine to be raised to optimize the combustion process.

In addition, for archaic carburetor engines equipped with mechanical systems gas supply, an increase in the compression ratio made it possible to compensate for the loss of power that occurred during the transition to gas. The fact is that gasoline and gases are mixed with air in the intake tract in completely different proportions, which is why when using propane-butane, and especially methane, the engine has to run on a significantly leaner mixture. As a result, the engine torque is reduced, leading to a power drop of 5-7% in the first case and 18-20% in the second. At the same time, on the graph of the external speed characteristics the shape of the torque curve of each particular motor remains unchanged. It simply shifts down the "Newton-meter axis".

However, for engines with electronic systems injection systems equipped with modern gas supply systems, all these recommendations and figures have almost no practical significance. Because, firstly, their compression ratio is already sufficient, and even for the transition to methane, the work of grinding the cylinder head is completely unjustified economically. And secondly, the processor of the gas equipment, coordinated with the electronics of the car, organizes the supply of fuel in such a way that at least half compensates for the above torque failure. In systems with direct injection and in gas-diesel engines, gas fuel in certain speed ranges is completely capable of raising torque.

In addition, the electronics clearly monitor the required ignition timing, which, when switching to gas, must be greater than for gasoline, all other things being equal. Gas fuel burns more slowly, which means that it needs to be ignited earlier. For the same reason, the thermal load on the valves and their seats increases. On the other hand, the shock load on the cylinder-piston group becomes smaller. In addition, winter start-up on methane is much more useful for it than on gasoline: gas does not wash oil off the cylinder walls. And in general, gas fuel does not contain metal aging catalysts, more complete combustion of fuel reduces exhaust toxicity and carbon deposits in cylinders.

Autonomous navigation

Perhaps the most notable disadvantage of gas car becomes its limited autonomy. Firstly, the consumption of gas fuel, if considered by volume, is more than gasoline, and even more so diesel fuel. And secondly, gas car is tied to the corresponding gas stations. Otherwise, the meaning of its transfer to alternative fuel begins to tend to zero. It is especially difficult for those who drive on methane. There are very few methane gas stations, and they are all tied to main gas pipelines. These are just small compressor stations on branches of the main pipe. In the late 80s - early 90s of the twentieth century in our country, they tried to actively convert transport to methane within the framework of the state program. It was then that most methane gas stations appeared. By 1993, 368 of them had been built, and since then this number, if at all, has grown only slightly. Most gas stations are located in the European part of the country near federal highways and cities. But at the same time, their location was determined not so much from the point of view of the convenience of motorists, but from the point of view of gas workers. Therefore, only in very rare cases, gas stations were located directly on the highway and almost never inside megacities. Almost everywhere, in order to refuel with methane, it is necessary to make a detour for several kilometers to some industrial zone. Therefore, when planning a long-distance route, these gas stations must be sought and remembered in advance. The only thing that is convenient in such a situation is the consistently high quality of fuel at any of the methane stations. Gas from the main gas pipeline is very problematic to dilute or spoil. Unless the filter or drying system at one of these gas stations can suddenly fail.

Propane-butane can be transported in tanks, and due to this property, the geography of gas stations for it is much wider. In some regions, you can refuel even in the farthest outback. But it doesn’t hurt to study the presence of propane stations on the upcoming route, so that their sudden absence on the highway does not become an unpleasant surprise. At the same time, liquefied gas always leaves a certain risk of getting into fuel out of season or simply of poor quality.

Characterized by a number of values. One of them is the compression ratio of the engine. It is important not to confuse it with compression - the value of the maximum pressure in the engine cylinder.

What is compression ratio

This degree is the ratio of the volume of the engine cylinder to the volume of the combustion chamber. Otherwise, we can say that the compression value is the ratio of the volume of free space above the piston when it is at bottom dead center, to the same volume when the piston is at the top point.

It was mentioned above that compression and compression ratio are not synonymous. The difference also applies to designations, if compression is measured in atmospheres, the compression ratio is written as a ratio, for example, 11:1, 10:1, and so on. Therefore, it is impossible to say exactly what the compression ratio in the engine is measured in - this is a "dimensionless" parameter that depends on other characteristics of the internal combustion engine.

Conventionally, the compression ratio can also be described as the difference between the pressure in the chamber when the mixture is supplied (or diesel fuel in the case of diesel engines) and when the fuel portion is ignited. This indicator depends on the model and type of engine and is due to its design. The compression ratio can be:

• high;
• low.

Compression calculation

Consider how to find out the compression ratio of an engine.

It is calculated by the formula:

Here, Vp means the working volume of an individual cylinder, and Vc is the value of the volume of the combustion chamber. The formula shows the importance of the camera volume value: if, for example, it is reduced, then the compression parameter will become larger. The same will happen in the case of an increase in the volume of the cylinder.

To find out the displacement, you need to know the cylinder diameter and piston stroke. The indicator is calculated by the formula:

Here D is the diameter and S is the piston stroke.

Illustration:

Since the combustion chamber has a complex shape, its volume is usually measured by pouring liquid into it. Knowing how much water fit in the chamber, you can determine its volume. For determination, it is convenient to use water because of the specific gravity of 1 gram per cubic meter. cm - how many grams are poured, so many "cubes" in the cylinder.

An alternative way to determine the compression ratio of an engine is to refer to its documentation.

What affects the compression ratio

It is important to understand what the compression ratio of the engine affects: compression and power directly depend on it. If you make the compression more, the power unit will receive greater efficiency, since the specific fuel consumption will decrease.

The compression ratio of a gasoline engine determines what octane rating it will consume. If the fuel is low octane, it will lead to the annoying phenomenon of detonation, and too high an octane number will cause a lack of power - a low compression engine simply cannot provide the necessary compression.

Table of the main ratios of compression ratios and recommended fuels for gasoline internal combustion engines:

Compression	Petrol
To 10	92
10.5-12	95
From 12	98

Interestingly, turbocharged gasoline engines run on fuel with a higher octane rating than similar naturally aspirated ICEs, so their compression ratio is higher.

Diesels have even more. Since high pressures develop in diesel internal combustion engines, this parameter will also be higher for them. Optimal compression ratio diesel engine is in the range from 18:1 to 22:1, depending on the unit.

Changing the aspect ratio

Why change degree?

In practice, this need rarely arises. You may need to change the compression:

• if desired, force the engine;
• if you need to adapt the power unit to work on non-standard gasoline for it, with an octane number different from the recommended one. For example, Soviet car owners did this, since there were no kits for converting a car to gas for sale, but there was a desire to save on gasoline;
• after an unsuccessful repair, in order to eliminate the consequences of incorrect intervention. This may be a thermal deformation of the cylinder head, after which milling is needed. After the compression ratio of the engine has been increased by removing a layer of metal, it becomes impossible to work on gasoline originally intended for it.

Sometimes the compression ratio is changed when converting cars to run on methane fuel. Methane has an octane number of 120, which requires increasing the compression for a number of gasoline cars, and lowering it for diesels (SG is in the range of 12-14).

Converting diesel to methane affects power and leads to some loss of power, which can be compensated by turbocharging. A turbocharged engine requires an additional compression reduction. It may be necessary to refine the electrics and sensors, replace the nozzles diesel engine on spark plugs, a new set of cylinder-piston group.

Forcing the engine

In order to produce more power or to be able to run on cheaper grades of fuel, the internal combustion engine can be boosted by changing the volume of the combustion chamber.

To obtain additional power, the engine should be boosted by increasing the compression ratio.

Important: a noticeable increase in power will be only on the engine that normally operates with a lower compression ratio. So, for example, if a 9:1 ICE is tuned to 10:1, it will produce more extra horsepower than a stock 12:1 engine boosted to 13:1.

The following methods are possible, how to increase the compression ratio of the engine:

• installation of a thin cylinder head gasket and refinement of the block head;
• cylinder bore.

By finalizing the cylinder head, they mean milling its lower part in contact with the block itself. The cylinder head becomes shorter, which reduces the volume of the combustion chamber and increases the compression ratio. The same happens when installing a thinner gasket.

Important: these manipulations may also require the installation of new pistons with enlarged valve recesses, since in some cases there is a risk of piston and valves meeting. The valve timing must be reconfigured.

Boring the BC also leads to the installation of new pistons to the appropriate diameter. As a result, the working volume increases and the compression ratio increases.

Deforcing for low-octane fuel

Such an operation is carried out when the issue of power is secondary, and the main task is to adapt the engine to another fuel. This is done by lowering the compression ratio, which allows the engine to run on low-octane gasoline without knocking. In addition, there is a certain financial savings on the cost of fuel.

Interesting: a similar solution is often used for carburetor engines of old cars. For modern injection ICEs with electronic control deforcing is highly discouraged.

The main way to reduce the compression ratio of the engine is to make the cylinder head gasket thicker. To do this, take two standard gaskets, between which an aluminum gasket-insert is made. As a result, the volume of the combustion chamber and the height of the cylinder head increase.

Some interesting facts

methanol engines racing cars have a compression ratio greater than 15:1. For comparison, standard carbureted engine consuming unleaded gasoline has a maximum compression ratio of 1.1:1.

Of the serial samples of engines on gasoline with a compression of 14: 1, there are samples from Mazda (Skyactiv-G series) on the market, which are installed, for example, on the CX-5. But their actual CO is in the range of 12, since these motors use the so-called "Atkinson cycle", when the mixture is compressed 12 times after the late closing of the valves. The efficiency of such engines is not measured by compression, but by the expansion ratio.

In the middle of the 20th century, in the world engine building, especially in the USA, there was a tendency to increase the compression ratio. So, by the 70s, the bulk of the samples of the American automobile industry had an SJ from 11 to 13: 1. But the regular operation of such internal combustion engines required the use of high-octane gasoline, which at that time could only be obtained by the ethylation process - the addition of tetraethyl lead, a highly toxic component. When new environmental standards appeared in the 1970s, ethylation began to be banned, and this led to the opposite trend - a decrease in coolant in serial engine models.

Modern engines have an automatic ignition angle control system that allows the internal combustion engine to operate on "non-native" fuel - for example, 92 instead of 95, and vice versa. The UOZ control system helps to avoid detonation and other unpleasant phenomena. If it is not there, then, for example, filling a high-octane gasoline engine that is not designed for such fuel can lose power and even fill the candles, since the ignition will be late. The situation can be corrected by manually setting the UOZ according to the instructions for a specific car model.