Power system (fuel system). The main differences between gasoline and diesel engines

4.11.2 Fuel for powering gasoline engines

It goes without saying that a gasoline engine requires gasoline to run.

Note
Gasoline is a light fuel oil and is a light liquid, which evaporates in air and is highly flammable.

Volatility, calorific value and antiknock quality are the main properties of gasoline. For the crowd, the defining characteristic is the latter.

Antiknock quality determines the possible value of the engine compression ratio.

Note
Knocking is the combustion of fuel at such a speed that it can be safely called an explosion (2000 m/s and more versus 20-40 m/s in case of normal combustion), which occurs at extremely high temperatures with a sharp increase in pressure in the cylinder. It occurs when using fuel with an inappropriate engine compression ratio and antiknock quality.

In case of detonation combustion of the mixture in the engine, sharp metal knocks and ringing are heard, explained by the impact of high-pressure sound waves on the walls of the combustion chambers, cylinders and piston tops and the occurrence of vibration in parts. At the same time, enhanced heat transfer and an increase in mechanical losses as a result of incomplete fuel combustion. The power and economy of the engine are sharply reduced.

Note
Long-term operation during detonation combustion can lead not only to increased wear of engine parts, but even to their breakdown or the creation of large defects in form of cracks and bending of parts with their subsequent destruction.

Therefore, if during the movement constant resonant metal blows are heard from the engine side, it is better to stop. Otherwise, there will be a possibility of major overhaul of the engine or its replacement.

Note
Knocking can be eliminated by reducing engine load (downshifting) and closing the throttle blade.

Gasoline octane number is an indicator characterizing the antiknock qualities of gasoline. The higher the octane number of gasoline, the less it detonates and the higher the compression ratio can be adopted for the engine.

Gasoline brands that are sold on the fuel market have their own designation. For example, A-95, where "A" tells us that gasoline is "automobile", and the number after the letter denotes the octane number.

4.11.3 Mixing and combustible mixture compositions

Mixing of fuel is the process of mixing finely atomized fuel with air.

It is necessary that the mixture being prepared, which is called the fuel-air mixture, satisfies two basic requirements:

1) the mixture, when ignited inside the engine cylinder, must burn very quickly, in a time interval measured in thousandths of a second, in order to ensure the appropriate gas pressure on the piston at the beginning of its working stroke;

2) gasoline in the air-fuel mixture must burn completely in order to provide the greatest heat release and increase the economy of the engine.

This mixture can burn quickly and completely, provided that gasoline and air are mixed in a strictly defined weight proportion, and there is a very thorough atomization and evaporation of gasoline in the air and their mixing.

In this case, each smallest particle of fuel will be surrounded by oxygen particles in the required amount, which will ensure the simultaneous rapid and complete combustion of the entire mixture.

Depending on the weight ratio of gasoline and air, the following types of mixtures are distinguished: normal, diluted, lean, enriched, and rich.

4.11.3.a Normal mix

A normal mixture is a mixture in which about 15 kg of air is needed for 1 kg of gasoline. Such amount of air is theoretically necessary for the complete combustion of fuel. On this mixture, the engine runs stably with average power and economy indicators.

4.11.3.b Lean mix 

Diluted mixture is a mixture in which there is an excess of air against the normal amount of fuel (approximately 16.5 kg, or even more, per 1 kg).

When operating on a diluted mixture, the engine power is somewhat reduced due to the deceleration of the combustion rate of the mixture. But the efficiency increases markedly, since less fuel is consumed.

4.11.3.c Poor mix

A рoor mixture is a mixture that has a significant excess of air. Due to the relatively large distance between the particles of gasoline that are sprayed in the air, the рoor mixture burns slowly and the gas pressure in the engine cylinder decreases. Due to the slow combustion of the mixture, most of the heat is absorbed by the walls of the cylinders and their cooling liquid, which causes the engine to overheat. The engine runs unstable on a рoor mixture. Its power decreases, and the specific fuel consumption (fuel consumption per unit of power) increases significantly.

4.11.3.d Enriched mixture

Enriched mixture is a mixture that has a slight lack of air if compared with the normal amount (about 13 kg per 1 kg of fuel). The combustion rate of the enriched mixture increases. As a result of this the pressure of the gases in the cylinder increases by the beginning of the working stroke of the piston. Therefore, when operating on an enriched mixture, the engine develops the highest power, but at the same time, a slightly increased fuel consumption is observed.

4.11.3.e Rich mix

A rich mixture is a mixture that has a significant lack of air compared to the normal amount. In such a mixture, due to a lack of oxygen, gasoline is not burned completely. This causes a decrease in engine power with significant fuel consumption. Unburned fuel particles in the form of carbon deposits are partially deposited inside the cylinders, and their main part is thrown into and out of the exhaust pipe in the form of black smoke. 

Pops and shots are created as a result of the afterburning of unburned fuel inside the exhaust pipe. This is an external sign of a strong enrichment of the mixture.

Based on the considered properties of various compositions of the combustible mixture, it can be concluded that if the engine, according to the operating conditions, should not develop full power (at medium loads), the diluted mixture is the most advantageous, since fuel consumption will significantly decrease. The resulting decrease in engine power during operation at partial load does not matter. At high loads, it is more expedient to work on an enriched mixture, since the engine develops the greatest power at the same time. A slightly increased fuel consumption due to the short-term operation of the engine in this mode does not cause a noticeable increase in total consumption over a long period of time.

Running on a lean or rich mixture, which will reduce the power and economy of the engine, is unacceptable.

4.11.4 The simplest carburetor

All the carburetors, even the most modern ones that ended their life in the mid-nineties of the twentieth century, basically work according to the principle, which can be described by the example of the simplest carburetor. It will be described and shown in Figure 4.47 b.

Figure 4.47b Simple carburetor.

A carburetor is a device, which mixes gasoline with air in a certain proportion and thoroughly sprays gasoline in the air.

Upon paying attention to Figure 4.47 b, we can say with confidence that, indeed, there is nothing complicated in the carburetor! It consists of the following parts: a float chamber with a float and a needle valve, a metering device with an orifice plug and a disperser, a mixing chamber with a diffuser, a throttle blade and air dampers. The carburetor mixing chamber is connected to the engine intake manifold.

The float chamber is used for maintaining a constant fuel level inside the disperser. Using a float with a needle valve, the fuel in the chamber and the disperser is maintained at a constant level - 1-1.5 mm above the lower end of the disperser. This level ensures easy suction of fuel from the disperser and eliminates the leakage of fuel from it when the carburetor is not working. The carburetor, shown in Figure 4.47b, receives fuel from the fuel tank by gravity due to the ubiquitous force of gravity on Earth.

At the moment of lowering the fuel level inside the chamber, the float descends and opens the needle valve. The fuel starts flowing into the float chamber. Upon reaching the normal level, the fuel will raise the float, which, using a needle valve, will cut off its flow into the float chamber.

4.11.4.a Spray

The disperser serves to supply fuel to the center of the diffuser, where it is sprayed. It is a thin tube, which enters the diffuser and communicates through the nozzle with the float chamber.

4.11.4.b Jet

The orifice plug doses the amount of fuel, which passes to the disperser. It is designed as a plug with a calibrated hole.

The mixing chamber serves to mix fuel with air. It is a short straight or curved pipe, which is connected at one end to the engine intake pipe, and at the other end to a diffuser, through which purified air passes into the carburetor.

4.11.4.c Diffuser

The diffuser provides an increase in the air flow rate at its center and creates a vacuum at the end of the disperser, which is necessary for sucking fuel from the disperser and better atomizing it.

With the help of throttle blade one can change the flow passage of the hole for the air-fuel mixture and thereby regulate the amount of this mixture coming from the carburetor to the engine cylinder. The engine power and the number of revolutions of the crankshaft change in accordance with the amount of mixture entering the engine.

4.11.4.d Air damper

With the help of air shutter one can reduce the flow passage for the air entering the carburetor, and thereby increase the vacuum in the mixing chamber, and therefore, increase the fuel supply. Air shutter is usually only used when starting the engine and is controlled from the driver's cab.

4.11.4.e Carburetor operation

Now let’s describe the principle of operation of the carburetor. When the engine crankshaft rotates during the intake strokes occurring in its cylinders, air passes through the carburetor mixing chamber. Inside the diffuser, the air flow rate increases significantly, and a vacuum is obtained near the end of the diffuser. In this case, the fuel from the disperser enters the mixing chamber in streams that are sprayed onto the smallest particles by the air passing at high speed. The fuel mixes with air, evaporates in it, and the resulting air-fuel mixture enters the engine cylinders through the intake manifold. The float chamber with the help of a float and a needle valve continuously maintains a normal fuel level inside the disperser (do not forget that we are describing a simple carburetor, in which there are no additional devices, such as an economizer, an econostat or an accelerating pump).

While driving, the driver essentially sets the carburetor throttle blade to various positions. More or less amount of the combustible mixture enters the engine cylinders. As a result of this the required engine power and vehicle speed are obtained.

4.11.5 Engine operating modes and carburetor requirements

In this section, we will consider various modes of operation of the carburetor, depending on the operating conditions of the engine. We will also provide the requirements for the functions of the carburetor and see how the structurally simplest carburetor should be transformed in order to fulfill all the tasks assigned to it.

The following main operating conditions of the engine are distinguished:

• start (speaks for itself);
•  idle run (a mode in which it is necessary to maintain idle speed, this is stated in a note);

Note
Idle RPM is the minimum RPM at which the engine can run stably without load. You have started the engine without any movement or impact on the accelerator pedal.

• medium loads (driving on an unloaded highway/road without much acceleration - evenly);
• full load (starting the movement of the car, drag racing and everything of this kind);
• fast transition from medium load to full load (mainly when overtaking).

Depending on the mode, it is necessary not only to supply a different amount of the air-fuel mixture to the engine cylinders, but also to use a different ratio of components to obtain it.

4.11.6 Start 

When starting the engine, more gasoline should flow into its cylinders. This is achieved due to a strong enrichment of the mixture as a result of increased fuel supply to the mixing chamber of the carburetor and to the walls of the intake manifold.

When the engine starts, the vacuum, which is created inside the carburetor diffuser is not enough for fuel to start flowing through the disperser. An air shutter is used for creating the vacuum, which is necessary for starting the engine. It is covered during start-up, as a result of which the required amount of fuel starts flowing through the disperser. Once the engine has started to run and warmed up, the air shutter is fully opened again.

4.11.7 Idle run

A small amount of fuel is required to operate at idle run. But the mixture must be rich enough to be sufficient for stable engine operation.

In the simple carburetor, which is designed for normal operation when the throttle blade is closed, for the engine to operate at low idle speed, the vacuum in the diffuser is reduced so much that fuel does not flow from the disperser at all. Therefore, an idle system is introduced, which allows the engine to operate regardless of the vacuum in diffuser.

4.11.8 Medium loads

At medium loads (from low loads up to 85% of full load), it is necessary to supply a different amount of air-fuel mixture, but always of the same type - a little diluted, in order to obtain the best efficiency indicators.

Note
Orifice plug is similar to a bolt, only without a head, element with a drilled through hole, the size of which is selected with high accuracy (it is called calibrated).

Note
In the simple carburetor, by selecting an orifice plug of the appropriate diameter and a diffuser, it is possible to obtain a mixture of the required composition only at a certain constant, for example, an average, position of the throttle blade.

With further opening of the shutter, the mixture prepared by the carburetor starts enriching. This happens because in this case the vacuum in the diffuser increases significantly. As a result of this, the resistance to fuel flowing out of the orifice plug becomes less than with a low vacuum in the diffuser. Therefore, the amount of fuel entering the mixing chamber increases, but not proportionally to the increase in the amount of passing air, which leads to an enrichment of the mixture. When the shutter is closed, the mixture starts diluting. In order to maintain approximately constant, the most advantageous composition of the mixture with different openings of the throttle blade at medium loads, i.e., to compensate for the mixture, a special device must be in the carburetor (for example, an economizer).

4.11.9 Full load 

At full engine load, the air-fuel mixture, which is supplied to its cylinders must be enriched. This is essential for obtaining the maximum power from the engine. The simple carburetor does not provide such level of enrichment. A special device, which is called an economizer, must be installed into the carburetor in order to fulfill this requirement.

When the throttle blade is opened quickly (when pressing the accelerator pedal), more gas must be supplied, enriching the mixture so that the engine quickly increases the number of revolutions, increasing power.

In case if the throttle blade is quickly opened in the simple carburetor, at the first moment, a strong dilution of the mixture is obtained. As a result of this the speed of acceleration (acceleration capability) of the engine deteriorates. This is explained by the fact that air, which has a lower density and has good mobility, when opening the shutter immediately rushes into the mixing chamber in a significant amount. Due to its higher density, the fuel is less mobile and does not have time to pass quickly through the orifice plug at the first moment. Therefore, the mixture becomes diluted. In order to increase the acceleration capability of the engine, a special device, which is called an accelerating pump, is installed into the carburetor.

Примечание
Thus, the simple carburetor under various operating conditions of the engine does not provide its power supply with an air-fuel mixture of the proper composition and must be equipped with additional devices: to compensate the mixture, for easy starting of the engine, for its idle run, for enriching the mixture at full load and for improving the acceleration capability of the engine (Figure 4.48).

Figure 4.48 Carburetor of VAZ 2108/2109 “Solex” car family.

1. Acceleration pump actuator arm; 
2. Starting arrangement diaphragm adjustment screw;
3. Starting arrangement diaphragm;
4. Starting arrangement air duct; 
5. Solenoid shut-off valve; 
6. Idle system fuel orifice plug; 
7. First chamber main air bleed; 
8. Idle system main air bleed; 
9. Air shutter; 
10. First chamber main dosing system disperser; 
11. Accelerator pump disperser with ball valve; 
12. Second chamber main dosing system disperser; 
13. Econostat disperser; 
14. Second chamber main air bleed; 
15. Second chamber transition air bleed; 
16. Float chamber balancing channel; 
17. Float chamber; 
18.  Fuel (needle) valve; 
19. Fuel return orifice connection; 
20. Mesh filter; 
21. Fuel supply connection hose; 
22. Diaphragm of economizer of power modes; 
23. Economizer fuel jet of power modes; 
24. Ball valve of economizer of power modes; 
25. Float; 
26. Fuel jet of econostat with tube; 
27. Second chamber transition fuel jet with tube; 
28. Second chamber emulsion tube; 
29. Second chamber main fuel jet; 
30. Outlet of the second chamber transition system; 
31. Throttle blades; 
32. Damping orifice plug; 
33. Throttle blades;
34. First chamber transition system slot; 
35.  Outlet of idle run system; 
36. Carburetor heating unit; 
37. Adjustment screw of “quality” composition of the idle system mixture; 
38. Engine breather system connection hose; 
39. Unloading unit connection hose to the vacuum regulator of the ignition distributor; 
40. Exhaust gas system recirculation unloading unit connection hose; 
41. First chamber main fuel jet; 
42. First chamber emulsion tube;
43.  Acceleration pump ball valve; 
44. Acceleration pump diaphragm; 
45. Acceleration pump push rod.

4.11.10 The main differences between gasoline and diesel engines

The most important difference between engines that use different types of fuel is that the air-fuel mixture in a gasoline engine is ignited by an electric discharge from a spark plug, and in a diesel engine it ignites spontaneously from highly heated air. The strokes are the same, but the phases are shifted according to the beginning and end of fuel injection.

In modern diesel engines, fuel is injected by a high-pressure nozzle directly into the cylinder. Thorough mixing of the air-fuel mixture is the main condition for normal operation.

So, a few major differences between a diesel engine and a gasoline engine are as follows:

1.  Ignition of the air-fuel mixture occurs due to the strong heating of the compressed air inside the cylinder.
2.  The combustion chamber is not located in the cylinder head, but in the piston (an example of how such a piston looks like is provided in the subsection "Piston" of section 4.7 "Engine Block and Crank Gear").
3. Two fuel pumps are installed in the fuel supply system: a low pressure fuel pump (LPHP or just a booster pump) and a high pressure fuel pump (HPFP), which creates sufficient pressure in the system for ensuring normal fuel atomization. The high pressure fuel pump is schematically shown in Figure 4.50.
4.  If a Common-Rail injection system is installed (the system design is shown in Figure 4.50), then the fuel is pumped into a special fuel rail by a high pressure pump (shown in Figure 4.49 and is a tube). Inside the rail, the pressure is maintained at the same level by the regulator (about 2000 bar, or even higher), and from the rail - it is supplied through the high-pressure fuel pipes to the fuel injectors.

Figure 4.49 Appearance of diesel engine fuel rail with Common-Rail system.

        5. A particulate filter is installed in the exhaust system of modern diesel engines.

Note
It is the soot, which is contained in the exhaust gases of diesel engines, that is one of the most dangerous components. And, according to scientists, it can lead to the formation of cancerous tumors.

       6. On some diesel engines, a urea supply system (the so-called SCR technology) is installed. It helps reducing emissions of harmful substances (namely, nitrogen oxides) when the engine is running.

Figure 4.50 Direct injection system of Common-Rail diesel engine.

4.11.11 Fuel

Diesel engines, as you might guess, use diesel fuel (often referred to as "heavy fuel"). Its quality is reflected by the cetane rating.

Note
The cetane rating is a characteristic of the flammability of diesel fuel, which determines the delay period of the combustion of the working mixture, i.e., the fresh charge (the time interval from fuel injection into the cylinder until the start of its combustion). The higher the cetane rating, the less the delay and the more calmly and smoothly the air-fuel mixture burns.

Heavy fuel oil contains wax products and sulfur. The more of them contains a diesel fuel, the worse, because sulfur reduces the ability to self-ignite, and wax products affect the operation of the fuel at low ambient temperatures. In case if a diesel fuel contains a high content of wax products, but does not contain the necessary additives, then after spending the night in an open parking lot at -15 °C, it will not be possible to start it, since the fuel itself in the pipes will turn into a kind of gelatin or solid oil. This, by the way, is one of the reasons for the installation of fuel filter heating systems and preheating glow plugs on many modern cars.