Petrol engine versus diesel engine

An engine is a lump of metal which makes the vehicles go zoom. Major types of engines are petrol and diesel. The engines required to run petrol and diesel are different because petrol and diesel are different types of fuel. Petrol is a highly volatile fuel and gets ignited very easily whereas diesel is comparatively heavy and dirtier fuel. We would be talking about 4 stroke engines only, the ones used in cars.

People who know me must be wondering why am I writing this article, i being a computer engineer. Hmmm, because, i am supposed to buy a car now and have been doing some research on them. The first question I came upon was whether a petrol or a diesel car. And, all i want to do is to share all that i have learned with you people. I am still unable to decide whether i should go for ford fiesta diesel or ford fiesta petrol, the petrol verson being a lot cheaper than the diesel version.

Lets start with what does a 4 stroke engine mean. It means that the engine has 4 strokes - inlet, compression, expansion and exhaust. Lets see stroke by stroke how both engines work

Petrol Engine


Diesel Engine


Inlet stroke
> In petrol engines the mixture of air and petrol is drawn in by the falling piston
> In diesel engines only air is drawn in by the falling piston

Compression stroke
> In petrol engine, the mixture is compressed upto about 1/8th to 1/12th of its original size.
> In diesel engine, only air is compressed upto about 1/14th to 1/25th of its original size.

Expansion stroke
> In petrol engine, the air and fuel mixture is ignited using a spark plug and burns expanding and forcing the piston down.
> In diesel engine, fuel is injected at a high pressure into the hot, compressed air in the cylinder, causing it to burn and force the piston down. No spark is required.

Exhause stroke
> In both petrol and diesel engines, the burned mixture of air and fuel is pushed out of the cylinder by the rising piston.

A diesel engine is also known as a "compression ignition" engine. Since the air is compressed to very high pressure raising its temperature and then diesel is injected in a very fine spray which causes the diesel to ignite and explode. Whereas a petrol engine is known as a "spark ignition" engine. Since a spark plug is required to ignite the mixture of petrol and air in the combustion chamber.

Diesel engine


Petrol engine


Lets also note down more differences of the same

• A diesel engine is more easily turbocharged than a petrol engine. A petrol engine cannot be easily turbocharged due to the fact that if the compression ratio and the pressure in the cylinder is to high during the inlet stroke, the mixture starts to burn to soon, while the piston is on its way up. The diesel engine has no fuel in the cylinder, thus letting the turbocharger suck as much air as it can without creating any problems. (A turbo charger is a simple air compressor which compresses air in the combustion chamber for burning). Some diesel engines also have an intercooler which helps in blowing cold and oxygen rich air in the combustion chamber.


• Electronic engine management not necessary in diesel engines. Some modern diesel engines are gaining electronically controlled injection pumps, but the vast majority of them out there have purely mechanical pumps. In fact no electricity is required to make a diesel engine run, except for a simple fuel cut off solenoid so that you can switch the thing off! If your alternator stops working, then you’re gonna get home in a diesel. This also means that a diesel engine does not have any ignition breakers, ignition coils, distributors and ignition wires to go bad. So a diesel engine should start no matter if it is dry or rainy or wet.


• Petrol destroys lubrication and burns the engine whereas diesel doesnt. So a diesel engine would last longer than a petrol engine.


• Petrol engines are lighter than diesel engines.


• Diesel engines have higher torque than petrol engines. What does this mean? Well, this means that a diesel engine would pull heavy loads easily than a petrol engine. Though the pickup of a petrol engine would be much more than that of a diesel engine, the diesel engine would be steady and carry heavier loads to longer distances.


• Diesel engines have better fuel efficiency as compared to petrol due to the fact that they have higher compression ratio.


• Diesel engines dont need an ignition system, which reduces their complexity. But they are more noisy and may require frequent maintenance as compared to petrol engines. Also they are more durable.


• Diesel engines may also need glow plugs in extreme cold conditions which heat up the cylinder so that a cold engine can start easily.


• And now the most important part, fuel economy. Diesel wins in both ways. Diesel engines give better mileage than petrol engines and In india diesel is much cheaper than petrol. So running on diesel would make you go farther at a lower cost than running on petrol.

Even after writing all this, i am still undecided where to go. Should i risk taking a diesel engine hoping that the maintenance costs would be bearable. Well, lets c what happens...

Interesting Info on Petrol and Diesel engines

I have been thinking of writing a post based on multiple topics, but I have been largely hit by the vast power cuts that have decreased the time I spend in front of computer.

Recently I have been discussing topics related to petrol (gasoline) and diesel engine with my friends. So, I thought of sharing some common questions and answers that may arise.

Please note that this post is not for mechanical or automobile engineering guys. Still, you may find some interesting info here.

1. What is the difference between a petrol and diesel engine?

Basically Petrol engines are spark ignition engines, whereas Diesel engines are Compression Ignition engines.

Petrol engines have carburetors to suck the fuel. The fuel is atomized (or vaporized) and is mixed with air. The air helps the fuel to burn. The air fuel mixture is let inside the cylinder, where it is burned by introducing a spark by means of spark igniter or spark plug. Thus they get the name Spark Ignition or SI engines.

Diesel engines don’t have carburetors, instead they have fuel injectors. Fuel Injectors inject fuel and supply it directly to the cylinder where burning occurs. Diesel Engines don’t have spark plugs. The compression ratio of diesel engine is more and also the auto ignition temperature of diesel is less than that of petrol. So, due to the compression, high pressure and temperature is created which burns the fuel. This is the principle of working of diesel engines and hence they have got the name Compression Ignition or CI engines.

2. Why do motor bikes have petrol engines and not diesel engines?

Some of the older model bikes had diesel engines. Diesel engines are bulky in nature. Carburetors are small, whereas fuel injectors are large and they have to be more accurate for proper functioning. Moreover higher compression ratio means higher vibration. Starting CI engines is quite difficult and particularly it’s hard during cold weather. So, diesel engines are not used in bikes.

3. What happens when diesel is used in petrol engines and vice versa?

Petrol has higher auto ignition temperature and it won’t burn at the compression ratio of a diesel engine without external help. So, the engine won’t run. Whereas when you use diesel in petrol engine, it’s dangerous. Diesel burns with more power and it may damage the engine partly or completely.

4. Why are diesel engine cars more expensive than petrol engine cars? Which is better to buy?

The components of diesel engine are expensive. As said earlier, they have to be manufactured with higher accuracy. Diesel engines have more components compared to petrol engines. This increases the total cost of the vehicle. The taxes imposed on diesel engine cars are also high.

On a long run and continuous usage diesel engine cars end up being cheap. This also depends on a lot of factors like the initial price difference the cars, cost of the fuel (currently petrol prices are increasing, but diesel prices are flat due to the extensive usage), cost of maintenance (diesel engines have higher maintenance costs), mileage, resale value and usage.

Also petrol engine cars are smoother to drive. You may have experienced it earlier. Bikes or petrol engine vehicles have lesser vibration whereas diesel engine vehicles such as buses have more vibration. This is due to the higher compression ratio of diesel engine. Hence the parts will be worn out soon too. With today’s technology the vibration effects are decreasing day by day.

5. Why is petrol red in color, whereas diesel yellowish in color?

Interestingly both the fuels have nearly the same color, except that petrol is dyed red in order to differentiate it from diesel. The color depends on countries, though it is mostly red. In some countries like South Africa, green color is used for unleaded petrol whereas red is used for leaded petrol.

That’s what I remember right now. If you have questions related to this or any other suggestions, please post in the comment section below.

Happy Reading!!!

Diesel engine exhaust classified as carcinogenic

The International Agency for Research on Cancer (IARC) classified diesel engine exhaust as carcinogenic to humans (Group 1), based on sufficient evidence that exposure is associated with an increased risk for lung cancer and limited evidence for an increased risk of bladder cancer.

Diesel engine exhaust arises from the combustion of diesel fuel in compression ignition engines. The sources of occupational exposure include motor vehicles, locomotives and diesel powered heavy equipment such as tractors and forklifts. Workers may either be exposed because they operate equipment that has a diesel engine or because their work takes them into situations where diesel engines are operating. Diesel exhaust also makes up an important part of the particulate air pollution found in cities and towns.

Emissions from diesel engines are a complex mixture of gasses, liquids and solids. IARC has identified that the carcinogenic components of diesel exhaust are probably associated with the particulate emissions and as a consequence it is common to use elemental carbon (EC), which makes up a significant fraction of these emissions, as a marker of exposure.

We have estimated that there are 3.6 million workers in the EU potentially exposed to diesel engine exhaust above background levels. The average occupational exposure level in Europe is probably around 13 μg/m³, although the levels are quite variable (geometric standard deviation of 2.7). There are very few countries that have an occupational exposure limit (OEL) for EC from diesel exhaust. In Austria, where there is a limit, it is set at 100 μg/m³; fewer than 5% of exposures in Europe exceed this figure. To have any impact on cancer risk the OEL would need to be much lower than this, perhaps less than 20 μg/m³.

Over about the last 20 years EU legislation has required engine manufacturers to reduce emissions from vehicles and, for example, the permitted particulate emissions from trucks and lorries will decreased by a factor of more than 50 times between 1992 and 2013. This emissions reduction has been driven by changes to the fuel and engine design.  Despite the increase in the proportion of diesel vehicles in the EU the exposure to diesel engine exhaust has probably been decreasing over the last 20 years by about 7% per annum. However, IARC say "existing fuels and vehicles without these modifications will take many years to be replaced, particularly in less developed countries, where regulatory measures are currently also less stringent. It is notable that many parts of the developing world lack regulatory standards, and data on the occurrence and impact of diesel exhaust are limited."

The IARC Working Group also concluded that gasoline exhaust was possibly carcinogenic to humans (Group 2B), a finding unchanged from the previous evaluation in 1989.