Study on Compression Ratio Optimization of M100 Methanol Engine

A turbocharged and intercooled diesel engine was transformed into an ignited M100 methanol engine by adding a spark plug to the cylinder head. A one-dimensional model of the methanol fuel engine was established using the bench test data as the boundary conditions, and the accuracy and accuracy of the model were verified. Methanol has better octane number and explosion resistance than traditional fuels, increasing the compression ratio of the engine will improve the power and fuel efficiency of the methanol engine. Based on the original compression ratio of 12, 8 compression ratio simulation schemes were selected, including 11.4, 11.7, 12, 12.3, 12.6, 12.9, 13.2, 13.5, etc., and the engine performance and the combustion state in the cylinder changed with the compression ratio were analyzed by simulation calculation, and the engine was optimized and improved. The results show that when the engine compression ratio reaches 12.9, the engine performance reaches the optimal value.


Introduction
Methanol fuel has a higher heat absorption capacity than other fuels of the same quality.As a result, when it enters the cylinder, it can reduce the temperature inside the cylinder and increase the intake volume, improve the charging efficiency, improve the engine's dynamic performance, reduce the exhaust temperature, and inhibit the NO X emission.Moreover, methanol has low carbon content and no sulfur.Compared with traditional fossil fuels, methanol is an ideal clean fuel.
The compression ratio of an engine has a significant impact on its performance.A suitable compression ratio can enhance the engine's power output and fuel efficiency.In other words, a reasonable compression ratio can improve the engine's overall performance and save fuel.Thus, affecting engine power, economy, and emission performance.Through simulation calculation, different compression ratio schemes are compared and analyzed, and the optimal compression ratio optimization scheme is obtained.
The engine's thermal efficiency has experienced a notable enhancement.Which can effectively optimize the engine performance, and the emission of CO and HC also has a significant decrease trend.Therefore, in this study on methanol fuel, the compression ratio of the reformed methanol engine was optimized to find the best compression ratio and explore the influence of different compression ratio

Compression Ratio Optimization Scheme
The magnitude of the compression ratio significantly affects the engine's performance, so in the process of optimizing the compression ratio, the representative speed in the external characteristic data can be selected to optimize the compression ratio.In the external characteristic data, 1000 r/min and 1400 r/min are the start and end points of the peak torque speed range of the engine, and 1900r/min is the maximum power point speed of the engine.Therefore, in the optimization simulation of compression ratio, these three speeds are selected for optimization.
Based on the original compression ratio of 12, the optimization scheme is to take a point every 0.3, which are respectively 11.4, 11.7, 12, 12.3, 12.6, 12.9, 13.2, 13.5, and other 8 compression ratio simulation schemes.Through simulation calculation, the engine performance, and the combustion condition in the cylinder change with the compression ratio are analyzed.In the calculation results, By comparing and analyzing key parameters such as power, torque, the amount of fuel used by the engine, the highest pressure reached in the engine's cylinders during operation, and knock coefficient, a compression ratio scheme is finally obtained which can make engine power and the economy more balanced.

Basic Parameters of the Engine
Based on a heavy-duty supercharged and intercooled diesel engine, this study transformed it into an ignited M100 methanol engine by adding spark plugs to the engine cylinder head.
Table 1 displays the primary characteristics of the engine.

Model Verification and Validation
To guarantee the credibility and precision of the simulation outcomes, we calibrated and checked the built one-dimensional model, including the cylinder pressure curve, power, torque, and effective fuel consumption rate.

Figure 6 Comparison of fuel consumption results
Figure 1 to Figure 6 show the comparison between simulation results and experiments.Through a comparison of the experimental and simulation results, it is found that the curve trend is consistent and consistent, and the maximum error is controlled within 5%, which indicates that the parameters and boundary conditions of the model are reliable and meet the calculation needs.The model can be used to simulate the methanol engine.

Compression Ratio Optimization Simulation
From Figure 7 to Figure 11, at the three rotational speeds of 1000 r/min, 1400 r/min, and 1900r/min, the power and torque of the engine gradually increase with the increased compression ratio, and the power performance increases obviously.
The primary cause is that as the compression ratio increase, the piston reaches the top dead center, The combustion chamber experiences a notable rise in pressure and temperature.This component speeds up the combustion rate of the mixture and reduces the ignition delay period.And increases the expansion ratio of the engine, so the engine power and torque are improved.According to the integration of knock induction time, it can also be seen that the knock tendency in the cylinder gradually increases as the compression ratio rises.
An increase in compression ratio leads to a compression ratio of 13.2 at 1000 r/min.The growth rate of power and torque slows down from about 1.26% to about 0.89%, which decreases by 0.37%.The decreasing rate of fuel consumption decreased from about 1.2% to about 0.86%, a decrease of 0.34%, and the increasing rate of thermal efficiency also decreased from about 1.21% to about 0.86%, a decrease of 0.35%.At this time, the detonation index reached about 94%, which was close to the critical point of detonation.When the compression ratio reaches 13.2 at 1000 r/min, the optimization trend of engine power performance and economy has slowed down significantly, and the continuous increase of compression ratio can no longer make the optimization effect more obvious, and the knocking tendency becomes more and more obvious.At 1400 r/min, when the compression ratio reaches 12.9, the growth rate of power and torque slows down from about 1.57% to about 1.01%, which is reduced by 0.56%.The decreasing rate of fuel consumption decreased from 1.38% to 0.98%, a decrease of 0.4%.The increasing rate of thermal efficiency also decreased from 1.39% to 0.97%, a decrease of 0.42%.The detonation index reached about 93%, which was close to the critical point of detonation.When the compression ratio reaches 12.9 at 1400 r/min, the optimization trend of engine power performance and economy has slowed down significantly, and further increasing the compression ratio can no longer make the optimization effect more obvious.Meanwhile, knocking will occur.This is because the torque at 1400 r/min is the maximum torque under external characteristic conditions.At this time, the engine torque is close to the maximum load, so the continuous increase of compression ratio will make the combustion in the cylinder more intense, resulting in the generation of a knocking phenomenon.
At 1900 r/min, when the compression ratio reaches 12.9, the growth rate of power and torque slows down from about 1.55% to about 1.02%, which decreases by 0.53%.The decrease rate of fuel consumption decreases from 1.43% to 0.91%, which is a 0.52% decrease.The increase rate of thermal efficiency also decreases from 1.42% to 0.99%, which is a 0.43% decrease.The detonation induction time integral reaches about 98%, which is close to the critical point of detonation.When the compression ratio reaches 12.9, the optimization trend of engine power performance and economy has slowed down significantly, and the continuous compression ratio augmentation can no longer make the optimization effect more obvious and will be accompanied by the phenomenon of knocking.
Figure 10 illustrates the efficiency of heat conversion of the methanol engine rises consistently with an increase in the compression ratio.Therefore, it is possible to increase the compression ratio as much as possible to enhance the efficiency of the methanol engine while ensuring the normal operation of the engine.Figure 12 and Figure 13 illustrate the correlation between compression ratio and NO X emissions and exhaust temperature.Specifically, with an increase in compression ratio, there is a gradual increase in NO X emissions.The increase in compression ratio results in a higher combustion reaction temperature and maximum pressure in the cylinder, creating a high-temperature and high-pressure environment that accelerates the generation of NO X , resulting in a gradual increase in NO X production.
As the compression ratio increases, the pre-vortex exhaust temperature at all three rotational speeds depicted in Figure 13 decreases.This is because, in the process of work done by the methanol engine, most of the heat is discharged from the engine in the form of exhaust gas.When the compression ratio increases, the expansion work done by the piston in the work stroke is also increasing, and more heat is used in the work.More and more heat is consumed when the work, so the heat taken away by the exhaust gas gradually decreases, and the exhaust temperature drops.

Figure 12 Optimization effect of compression ratio on NOx emission
Figure 13 Influence of compression ratio on exhaust temperature before the vortex

Conclusion
Through the simulation calculation of the compression ratio scheme of the methanol engine at three speeds, the following conclusions can be drawn: The knocking index indicates that the knocking tendency in the cylinder increases gradually as the compression ratio rises.At 1400 r/min and 1900 r/min, slight knocking occurs when the compression ratio reaches 13.2.As the compression ratio increases, engine power performance improves while fuel consumption decreases.However, this comes at the cost of deteriorating emission performance.
According to the calculation results of the knock induction index, the compression ratio of 12.9 was selected as the optimization target of the model compression ratio.Currently, the methanol engine experienced notable enhancements in its power output, overall performance, and fuel efficiency.

Figure 1 Figure 2 3 Figure 3 Figure 4 Figure 5
Figure 1 Comparison of pressure calibration results in 1000 r/min cylinder

Figure 7 Figure 8 Figure 9 Figure 11
Figure 7 Influence of compression ratio on power

Table 1
[1] Ravikumar Ramegouda, Antony Alappath Joseph.Effect of Compression Ratio on Performance and Emission Characteristics of Dual Spark Plug Ignition Engine Fueled With n-Butanol as Additive Fuel [J].International Journal of Renewable Energy Development, 2021, 10 (1).