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Causes of low efficiency in gasoline engines

Today, the internal combustion engine can achieve high efficiency of up to 40-52%, while the classical piston-type steam engine only achieves about 16%, steam turbine from 22-28%, and gas turbine only up to 30%. In order to achieve high performance, the engine must be affected by factors such as intake and exhaust, ignition control, energy conversion at the stroke, etc. To better understand the causes of performance low in gasoline engines, find out in this article.
Through many years of research and development, internal combustion engines are equipped with many new technologies for safe operation and fuel economy. The weight of the engine is made lighter to suit different vehicles but still meets its capacity and features.

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Although the motor is very efficient, it is not very efficient in some other operating modes, especially in the low load mode.

In the full load mode, the work generated during the expansion period is high because the throttle valve is fully open and the amount of air sucked into the cylinder is high, while the suction power consumption is low because the gas resistance of the throttle valve is low, so the efficiency is low. It will be high.

In partial load mode, the throttle is controlled very small because of the small load condition, so the amount of air sucked into the cylinder is small, and the work produced during the expansion period is small, while the suction power consumption is high because of the high resistance. The throttle has a high gas flow, so the work efficiency is low in high load condition.

Example. When the thermodynamic efficiency of a gasoline engine is calculated with a compression ratio ε = 9-10, it is usually only about 60% for an ideal cycle.

Influenced by the intake process.

During the intake process, the piston moves from TDC to BDC, creating a force that draws air into the combustion chamber. To do this, the piston must consume a certain amount of work, called pumping losses.

If in high load mode, the throttle is fully open, then air from outside is easily sucked in, but still obstructed by the intake valve, then the work consumed to suck in air is very little.

On the contrary, in low load mode, the throttle is only partially open. Due to the obstruction of the throttle, the piston performing the suction process is subjected to a considerable force, which acts on the top of the piston and makes the suction process much more intense.

In addition, the air intake is limited by the volume and pressure of the combustion chamber. However, the performance of the engine depends on the amount of air entering the cylinder, so increasing the intake volume can compensate for the losses due to lost work.

The exhaust process is the process that the piston must perform to push out combustible material. When the exhaust valve opens early at the beginning of the exhaust process, then combustion expansion is not completed. Part of the work of the combustion process is consumed in order to push the exhaust gases out.

by the residual gases.

The main requirement of the exhaust process is to push all combustion products out of the combustion chamber, but this is completely impossible. The residual combustible material is called residual gas and the amount of this material has a great influence on the charging and combustion process of the next cycle.

The residual air in the intake stroke takes up a portion of the cylinder volume, making the amount of air drawn in much less. During expansion and combustion, the fuel mixes with impurities, resulting in a wrong air-to-air ratio and the quality of the combustion process will be significantly reduced.

Effects due to heat transfer in the compression process.

During the compression process, the piston compresses the liquid and the volume changes, so part of the compression work is converted into heat. The heat that is converted is lost to the outside through the cylinder walls due to the engine cooler, which means that part of the converted mechanical energy is also lost and the work produced in this process is incomplete.

Affected by air leakage.

Inside the engine, the piston moves inside the cylinder, surrounded by rings and lubricant. The rings are responsible for sealing the combustion chamber so that combustible gases do not enter the bottom of the oil and oil is not allowed to enter the combustion chamber. But the fact is that when the engine is running, there is always a certain amount of fluid leaking out of the cylinder through the rings, especially during the compression process.

This is called air leakage. This phenomenon reduces the pressure in the compression process as well as the compression medium, resulting in a reduction of engine power in the compression process.

Affected by air quality.

The efficiency of the engine depends mainly on the quality of the combustion process. Because this is the only useful process for the engine to produce work. The combustion performance is determined by the composition of the carburetor, the formation of which has a decisive influence on the combustion process and thus on the economy, efficiency, smoothness and exhaust quality of the engine.

During operation, the engine requires carburetor components in each different operating mode. The ratio of air to fuel is referred to as A/FR.

Table of air-fuel ratios in different operating modes.

A/FR . Operating Mode

Start at low temperature (0oC) 1:1

Start at normal temperature (20oC) 5:1

No load 11:1

Deceleration 12-13:1

Acceleration 8:1

Maximum power 12-13:1

Average speed 16-18:1

If the fuel mixture is too rich, when combustion occurs there will not be enough oxygen to fully ignite the mixture. Therefore, not enough energy is produced and the combustion products will produce soot and harmful substances such as CO, HC …… , and the efficiency will be reduced.

On the contrary, the mixture is too poor, there will be too thin, so the combustion process does not occur, so toxic substances such as CO will be produced, and the efficiency will be reduced.

6. Affected by the burning rate of the fuel.

In the ideal cycle of an internal combustion engine, the combustion time is zero and the combustion rate is extremely fast.

From the moment the spark plug starts to ignite, the fuel starts to burn until it is completely burned. If the combustion rate is too slow, the combustion process will continue until the next period of the engine, reducing the power of the engine.

In practice, the burning rate of the fuel should match the engine rotation speed and load. Then the engine rotation speed and load are always changing, so to change the combustion speed you must change the ignition time, in other words, if the engine is at high speed, you must change the early ignition angle.

If the early ignition angle is too large, it will cause combustion and compression at the same time, which will consume compression and make the engine hot. On the contrary, the early ignition angle is too small, so that the combustion process continues on the expansion line, the exhaust gas temperature is high, the engine heats up and the efficiency is poor.

Affected by the compression ratio of gasoline engines.

Compression ratio is a parameter that directly affects the performance of the engine. As the compression ratio increases, the efficiency of the engine will also increase. However, the higher the compression ratio of a gasoline engine, the greater the likelihood of fire and deflagration (except in Mazda).

In addition, engines with higher compression ratios will strive to produce more power because the higher pressure and temperature during compression results in higher rates of combustion and pressure rise.

Influenced by the ignition system.

Spark plugs are needed to produce a spark in the final stages of compression. If the spark plug ignites too early or too late, the engine will consume more fuel, while increasing engine power and decreasing engine efficiency.

The position of the spark plug greatly affects the performance of the engine.

Specifically, the location of the spark plug in the combustion chamber has a significant impact on the propensity for detonation.

Placing the spark plug close to the intake and exhaust valves increases the likelihood of high carburetor temperatures at the end of the flame stroke, which is caused by the high temperature of the exhaust valves, thereby increasing the propensity for detonation.

Therefore, it is necessary to place the spark plug close to the middle region of the combustion chamber, near the hottest part of the combustion chamber, thus shortening the flame film stroke on the one hand and reducing the temperature of the carburetor block in the end region of the combustion chamber on the other. The flame film reduces the tendency to detonate.

In addition, the energy of the spark must also be sufficient to ignite the mixture in the engine cylinder.

Other motor-related losses.

Mechanical losses include the following components.

Frictional loss of work. The work required per cycle to overcome the friction caused by the relative motion of the engine components.

This work loss includes losses between the rings, piston body and cylinder walls, losses in the joints, connecting rod big ends, crankshaft and camshaft bearings, losses in the valve train, gears, belts and camshaft drive belts, and other components.

The power of the drive auxiliary mechanism. The work required to drive auxiliary mechanisms (such as pumps, fans, generators, etc.) for each cycle. Usually only the auxiliary mechanisms required to run the engine are included.

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