Gasoline vs. Diesel Engines: Back and Forth

With the growing concern on climate change, the common question in today’s automotive industry is “what is the best fuel that produces less CO2 emissions with excellent performance?” Vehicle manufacturers spend more and more time and money on research to answer the puzzling question: should we design vehicles with gasoline or diesel engines? For example Volkswagen cars produced with gasoline and Diesel engines (Figure 1). However, the answer to this question is difficult because each of these two options has its very own advantages that may or may not attract customers. For example, diesel engines have better fuel economy. On the other hand, gasoline engines provide way more power than diesel engines.

Properties

To understand the scenario in better way, it is important to review some of the important properties of diesel and gasoline fuels. Gasoline fuel is lighter than diesel, and hence it evaporates faster. Diesel contains more carbon atoms in longer chains than gasoline does (gasoline is typically C9H20, while diesel fuel is typically C14H30). It takes less refining to create diesel fuel, which is why it used to be cheaper than gasoline. Due to its heavy carbon chain, diesel fuel has a higher energy density than gasoline. On average, 1 litre of diesel fuel contains approximately 40.8×106 Joules, while 1 litre of gasoline contains 34.7×106 Joules.

(a)                                                                         (b)

Figure 1: Volkswagen produced vehicles with different fuel engines

(a) Gasoline; (b) Diesel

Aspects

The design of diesel engine is based on the concept of the high-compression ratios in the combustion cylinders. Therefore, they must be built to withstand heavier task than gasoline engines. Major parts of diesel engine are made thicker and stronger; increased weight of heavy-duty components is one of the diesel engine disadvantages. A diesel engine can weigh few hundred kilograms more than a comparable gasoline model. However, the higher thermal energy of diesel, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines, and why they are preferred in vehicles where high torque is required, such as trucks.

The last decade witnessed serious improvements in noise isolation and vibration in diesel engines. However, they are still louder and shake more than gasoline engines. Major difference in stability and noise is noticeable at low speed or even idle condition; on the highway, there’s little difference between the two though.

In diesel engine the high compression ratio is used as the source for spontaneous ignition once the fuel is injected into the cylinder. The high compression creates enough heat to ignite the heavy fuel. Therefore, these engines don’t have spark plugs like gasoline engines. However, a problem appears when it’s cold (i.e. -30oC), the air isn’t hot enough to ignite the fuel. An improvement for this condition has been achieved by using a computer that senses cylinder temperature and injects the fuel later in the engine rotation. On the other hand, the spark plug in gasoline engines does the job of ignition even at low temperature conditions.

 

Figure 2: Diesel fuel emissions

Emissions

The progress in refining industry technologies, together with environmental regulations imposed by governments encouraged the improvement of diesel and gasoline fuel quality from environmental point of view. As diesel and gasoline fuels have different chemical composition, their emission composition comes different. Unfortunately, burned diesel fuel still smells much worse than burned gasoline. Beyond the smelly tailpipe, diesel lags behind gasoline in the areas of NOx and particulate matter emissions. NOx is a greenhouse gas and one of the components of smog; it is one of the evaluation criteria for internal combustion engines. Meanwhile the particulate matter causes the black soot seen emanating from diesel-vehicle tailpipes (Figure 2). On the contrary, it is well known that passenger vehicles with diesel engines emit, depending on their condition, about 20% less of CO2 per kilometre than the same vehicle type with gasoline engines.

The U.S. Government’s Environmental Protection Agency (EPA) lists the average CO2 emissions from 1 litre of gasoline as 2.3 kg of carbon. The average carbon emission from 1 litre of diesel fuel is listed as 2.6 kg of carbon. Both gasoline and diesel give a net climate warming effect. However, diesel causes more warming than gasoline in the first decade after emission into the atmosphere, while gasoline causes most warming after that period. Therefore, the assessment of impact on environment depends on the emphasis whether it is on short-term or long-term basis. If the emphasis is on short-term climate changes or the rate of warming, diesel comes out worse. On the other hand, when the emphasis is on more long-term climate changes, diesel clearly comes out best due to lower CO2 emissions per kilometre driven.

In final, it can be concluded that both diesel and gasoline are still the main source of power for more than 90% of today’s vehicles, including here personal cars, public transportation, industrial and commercial vehicles. But even worse, public transportation produces almost a third of the overall CO2 output coming from gasoline- or diesel-powered vehicles while personal cars produced more than 60% of all CO2 emissions.
 

 

To cite this article, please use following information:

(use the given format or any standard citation format)

Ridha, F., Gasoline vs. Diesel Engines: Back and Forth, ChE Thoughts 2 (1), 6-7, 2011.

 

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