An engine is a machine designed to transform energy into convenient mechanical motion. Heat engines, including external combustion engines and internal combustion engines, burns fuel to produce heat that then creates motion. This paper discusses petrol and diesel engines and compares the efficiency of the two engines.
Diesel Combustion Engine
A diesel engine, also referred to as compression ignition engine is an engine with internal combustion. It makes use of the heated compression to initiate ignition. Further, it burns fuel that has been injected in the combustion chamber. Diesel engines have the highest thermal efficiency at any standards, external and internal combustion engines because of its high compression ratio. Low-speed diesel engines are mostly used in ships where the overall engine’s weight is unimportant and have thermal efficiency that exceeds 50% (Rajput, 2005, p34).
The manufacture of diesel engines uses four-stroke and two-stoke versions. Originally, they were used as a more effective replacement for stationary steam engines. They have been used in ships and submarines since the 1910s, in the 1930s they had developed and were used in trucks, heavy equipment, locomotives, and electric generating plants (Rajput, 2005, p. 33).
Petrol Combustion Engine
A petrol combustion engine is also known as a gasoline engine. It is an internal combustion engine with spark ignition. Petrol engines were invented by a German inventor, Nicolas August Otto in 1876. Nicolas August Otto`s invention uses a four-stroke combustion principle. This is also referred to as the Otto Cycle. Up to date, his principle is considered as the most basic principle in most car engines ( Chen, 2008, p. 11).
In most petrol engines, the air and fuel are normally pre-mixed before compression. Initially, the pre-mixing process was done in a carburetor but currently it is done through electronically controlled fuel injection, with exceptions in small engines where the complication or cost of electronics does not rationalize the added engine efficiency (Stewart, 2013).
Diesel Engines vs. Petrol Engines
Petrol and diesel engines are similar. This is because they are both internal combustion engines which are designed to convert chemical energy in fuels to mechanical energy. The mechanical energy moves pistons inside cylinders. These pistons are connected to the crankshaft. The linear motion caused by the up and down movement of the pistons creates a rotary motion. This motion is essential as it is needed to turn the wheels of a car ( Chen, 2008, p. 13).
The main difference that exists between petrol and diesel engine is how they convert fuel to energy through a series of combustion or small explosions. In a petrol engine, fuel and air are mixed then compressed by piston and finally ignited by sparks from spark plugs. However, in a diesel engine, air is first compressed then the fuel is injected. The air heats up when it is compressed causing the fuel to ignite (Stewart, 2013).
Additionally, the other significant difference between the two engines is in the injection process. Most engines use a carburetor or port injection. Port injection systems inject fuel prior to the intake stroke, outside the cylinder while a carburetor mixes fuel and air before the air enters the cylinder. In car engines, therefore, fuel is loaded in the cylinder through the intake stroke then compressed. The compression process of the air and fuel mixture constrains the compression ratio in the engine, if air is compressed too much, the air/fuel mixture spontaneously ignites causing the engine to knock. This causes excessive heat and can damage the engine. Diesel engines, unlike petrol engines, use direct fuel injection where fuel is injected directly into the cylinder (Rakopoulos & Giakoumis, 2009).
Gasoline engines run at high speeds than diesel engines, partly due to their lighter pistons, crankshafts and connecting rods and also because petrol burns faster than diesel. Petrol engines tend to have much shorter strokes. Pistons in a petrol engine, therefore, move up and down quicker than a diesel engine. However, it has a low efficiency than diesel engines because of its low compression ratio (Rajput, 2005, p. 55).
In relation to the environment, both engines have pros and cons. Diesel engines emit smaller amounts of hydrocarbons, carbon monoxide and carbon dioxide, therefore, they have less negative impacts to the environment. On the other hand, it emits higher amounts of nitrogen compounds as a result of diesel combustion, these leads to smog, acid rain and poor health conditions (Rakopoulos & Giakoumis, 2009, p. 104).
Diesel engines have a higher torque level than gasoline engines because of the difference in air intake requirements. Diesel engines take in as much air as they possibly can while petrol engines to limit air quantity. They can also bear heavy loads because they do not have the knocking effect; these make them more efficient than petrol engines ( Chen, 2008, p. 12).
Comparison criteria between the engines are more substantial when they are centered on dimensional variables. For example, fitting comparison criteria can be the efficiency of the engine based on the compression ratio keeping other dimensionless relationships constant between operating parameters. With respect to the engine’s power, evaluation criteria may be the ratio between BMEP and the pressure characteristic of the cycle. The ratio represents normalized power with respect to the rotation frequency, displacement and pressure. Maximum pressure of these cycles determines material thickness of the engine and its weight. If the pressure is selected to be a reference variable, the ratio above will give an idea about the power produced relative to the weight, size and speed.
Chen, G. (2008). Study of Fuel Temperature Effects on Fuel Injection, Combustion, and Emissions of Direct-Injection Diesel Engines. Journal of Engineering for Gas Turbines and Power, 131(2), 10-13.
Rajput, K. R. (2005). Internal Combustion Engines. london: london publishers.
Rakopoulos, C. D., & Giakoumis, E. G. (2009). Diesel Engine Transient Operation. London: Springer Verlag London Limited.
Stewart, J. (2013, October 2). BBC. Retrieved February 4, 2014, from http://www.bbc.com/future/story/20131001-pushing-car-engines-to-new-limits