Why diesel engine is much more efficient than petrol engine?

 Do you know why 4stroke 6cylinder diesel engine is much more efficient than petrol engine? I think the reasons are as follows.

Theoretical cycle

From the theoretical point of view of thermodynamics, the basic principle to improve the engine power and economy is to improve the energy quality after combustion effect heating, which means to increase the average temperature of the working medium in the heating process and reduce the average temperature of the heat release process to the environment as much as possible under the same heating amount. Under the current circulation mode, there are three main implementation directions:

l Increase the compression ratio of the engine as much as possible under allowable conditions.

l Reasonably organize combustion and improve the constant volume of circulation heating. The combustion heating center is close to the TDC by reducing the pre-expansion ratio of the cycle and selecting the combustion starting point phase reasonably.

l Ensure that the working medium has a high isentropic index.

The actual 4stroke electric starting diesel engine is closer to the isobaric cycle than the petrol engine under the rated condition. Because the compression ratio of diesel engine is much higher than that of petrol engine and its maximum combustion pressure is much higher than that of petrol engine, its thermal efficiency is higher than that of petrol engine. The diesel engine burns at the edge of spray after compression ignition. When the load is reduced, the injection time is shortened but the change of isochoric combustion in the initial stage is very small, which is equivalent to the constant pressure rise ratio and the decrease of pre-expansion ratio. Petrol engine is a kind of flame spreading combustion after ignition. No matter how the load changes, the flame propagation distance does not change.

The opposite trend of petrol engine and 4stroke electric starting diesel engine makes the difference of fuel consumption rate between the two engines in low and medium load further widen. This conclusion is in accordance with the actual situation. Under the rated condition, the effective fuel consumption rate of diesel engine is 15% - 25% lower than that of petrol engine.

Comparison of thermal efficiency between petrol engine and diesel engine under ideal cycle condition

Considering the real working fluid characteristics, the gap between the thermal efficiency of petrol engine and high speed 6cylinder diesel engine is further increased. 

1) Factors of widening gap at high load

Under the condition of high load, the mixture of petrol engine is relatively rich and the excess air coefficient is 0.8-1.0; the average excess air coefficient of powerful 12cylinder diesel engine is 1.3-2.0 and the mixture is generally lean.

Although the compression ratio of the petrol engine is low, the maximum combustion temperature of the petrol engine is much higher than that of the diesel engine due to its rich mixture and high constant volume. In addition, the residual emission coefficient of petrol engine is higher than that of diesel engine. These two reasons make the isentropic index of petrol engine smaller than that of powerful 12cylinder diesel engine. With the increase of high temperature decomposition, the thermal efficiency of petrol engine is much lower than that of diesel engine. Although the molecular variation coefficient of petrol engine is higher than that of diesel engine, its influence is small.

2） Factors for further expansion of the gap at low load

Because of the difference of mixture formation and load regulation, the lower the load, the smaller the excess air coefficient. Diesel engines further expand the gap between the thermal efficiency. The petrol engine regulates the load. The lower the load, the less air intake and the higher the residual exhaust coefficient. The coefficient of residual exhaust gas of high speed 6cylinder diesel engine generator remains unchanged at low load.

The temperature difference of working medium after combustion is smaller than that of diesel engine at low load, which means that the combustion temperature of petrol engine is still higher at low load. Although the amount of mixed gas entering the cylinder at low load is small, the calorific value of the unit mass working medium does not change, so the temperature drop of the working medium after combustion is not large. The high speed 6cylinder diesel engine generator is a quality regulated load. The calorific value of unit mass working medium decreases with the increase of excess air coefficient at low load. This factor makes the temperature difference between petrol engine and diesel engine more significant at low load, which will cause the difference of thermal efficiency between them.

By comparing the theoretical cycle and ideal cycle thermal efficiency of petrol engine and new motor 6cylinder diesel engine generator, the essential reason of the influence of two different combustion modes on thermal efficiency is explained theoretically. In recent years, the emergence of GDI and HCCI petrol engine is the outstanding embodiment of these cycle theory applications.