The Efficiency of Petrol Engines: An In-Depth Analysis

Petrol engines, ubiquitously found in today's automobiles, operate at significantly lower efficiency levels compared to diesel engines. Understanding why this is the case is crucial for anyone interested in automotive engineering or sustainability in transportation.

Overview of Thermal Efficiency in Petrol Engines

As a general rule, out of 100 energy units input into the fuel, only about 15-20% of that energy actually results in useful work (such as turning the crankshaft). Approximately 50-65% of the energy is lost as heat through the exhaust and cooling system, and around 20-30% is lost due to friction in moving parts, such as the engine's internal components, generators, and air intake and exhaust systems.

Factors Affecting Efficiency: Compression Ratio and Fuel Type

The efficiency of a petrol engine is closely tied to its compression ratio, which is the same as its expansion ratio in many cases. The compression ratio of petrol engines typically ranges from 8:1 to 10:1. Higher compression ratios are desirable as they allow for more efficient energy conversion, but they also require high-octane fuel to prevent detonation, which is self-ignition of the fuel before the spark plug fires. This constraint limits the overall efficiency of petrol engines.

In contrast, diesel engines operate with much higher compression ratios, typically ranging from 14:1 to 25:1. The slow-burning nature of diesel fuel allows it to be ignited without the need for spark plugs, and the resulting higher compression and expansion ratios significantly increase its efficiency up to 45%. This makes diesel engines more efficient for certain applications, especially those requiring higher power output.

Impact of Oxygen Availability on Fuel Combustion

A limitation of petrol engines is the scarcity of oxygen in the air. Air, by mass, is only about 21% oxygen. Therefore, the available oxygen in the air is often insufficient to fully burn the fuel within the cylinder, resulting in some portions of the fuel being left unburnt. Additionally, at high RPMs, the fuel has less time to burn completely, further contributing to unburnt fuel escaping into the atmosphere through the exhaust. This issue has been partially addressed by the advent of Variable Valve Timing Intelligence (VVTI) technology, which allows engines to utilize fuel more efficiently at high RPMs by adjusting valve opening and closing timings.

Conclusion

In summary, the lower efficiency of petrol engines compared to diesel engines is primarily due to factors such as the compression ratio, the need for high-octane fuel to prevent detonation, and the limited availability of oxygen in the air. Advances in technology, such as VVTI, continue to address these challenges, but diesel engines remain more efficient for many applications.