Wednesday, October 18, 2006

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...


Anonymous said...

Hi Jayant, we bought a Diesel Citroen C4 in June this year, and I do not regret doing so, how about 4.7 ltrs per 100K, diesel cost about 20cents more than petrol, in Aust. so on a sixty ltr tank of diesel i get about 1250 k driving, a 1.5/2.0 ltr petrol 4 cyl car would use about double my usage, I will never go back to petrol. tony

Ananth said...


I am planning to buy a car, but haven't decided whether to go for petrol or diesel (or gas!). Your post is very useful. And considering the fuel price diesel seems irresistible for India.

By the way, just curious to know which car you went for... :)


baltej said...

hi jayant,
thanks atleast i m bit clear about the basic difference between the two.
but jayant there r two virtues of petrol over deisel (wwhen we talk abt
cars) which makes petrol engine my favourirte..
1. smoothness of drive.(something we all dire for in our cars).
2. reaction.( or pick up).it is the reaction of our cars at expressways that we rely on ..hence more safe drives.
However economy of fuel is an irk..
thanks anyway ur article has been of gr8 info value

Will said...

Hi Jayant,

Thanks for your article. I recently traded in my Seat Leon Cupra which was a wonderful car to drive. It is a 1.8 Turbo 24 valve engine. The problem was it was being a bit thirsty with amongst some other problems cause i had it for years from new.

I traded the Seat in for a Honda Civic 2.2CDTI which is a lovely car. Designed nicely. However, the drivin experience is very different. The Seat is faster, more responsive and the engine is throaty (its the same engine as the old Audi TT) and it gives you 180 bhp. The Civic is a civilised drive and you can hear the diesel engine when you initially start the car and also in the 1st and 2nd gears. However, once you start driving you are fine and dont notice it.

I'm in the UK and petrol prices are about 1.18 per litre compared to diesels 1.30 per litre.

I would opt to have 2 cars 1 for economy and the other for pure driving experience.

Unknown said...

u better go with ford fiesta diesel which has good economy.v r having ford ikon flair a petrol engine.its difficult 2 maintain a petrol engine.a great disadvantage in ford fiesta diesel is that its getting vibrated when u reach 110 and has now launched diesel ikon which has the same engine of fiesta. it has even greater mileage than fiesta. there is no anty vibration complaint u better go with this

Unknown said...

hey jayant, u really educated me man.. good job.. plz do throw som light on the electronically controlled injction pumps when time permits... thanx buddy !!

kiran@bglr said...

I really appreciate for your valuable content which educated my existing querry.

ajmer-the heart of rajasthan said...

hey jayant uhv explaind a mechnical the most appropriate and concise manner...and u being from computer appreciable....thanx buddy..
we are going to buy dezire swift diesel vry soon any comments on it??

Krishna Kodoth said...

I'm goin thru Madness...Still cant decide!

My Heart Says Fusion + PETROL
My Wallet Says Fusion + DIESEL

Any thought buddies?....

Fishing Rules said...

excellent article

Anonymous said...

Your annual mileage/kilometerage and the price difference in the fuels should be the deciding factor in the diesel/petrol petrol debate.

In France where diesel is cheaper there's no question. I live in Switzerland and the extra cost of diesel means that with my annual average of 13,500km /year I would need 12 years to show an advantage in the extra cost of a diesel version.

But diesels are cleaner and for eg, the 2010 VW Polo puts the diesel version on econ category A, and the petrols in B.

And if I keep a car for say 10 years what will the fuel costs be in 2019?

Anonymous said...

I would like to share my views in what engine (petrol or diesel) you are going to select. I would suggest that if you want to get a car, ask your self how often are you going to use it. By knowing it, then you can weigh things like the maintenance cost, fuel cost, engine noise, etc. Maintenance cost for diesel engine will be more compared to petrol engine. if you frequently travelling at a far distance with much load, you may consider diesel engine.

Anonymous said...

one question to everyone:- which is environmentally friendlier? Diesel Engine or Petrol Engine?

gamegeek said...

petrol engine is environment friendly cause the emissions are less - petrol is a much cleaner fuel as compared to diesel

Ayuni said...

Hi.. your post is very-very useful to me as i am taking thermal and fluid machine subject this semester.. which is compulsory subject in engineering.. Thank You..

Anonymous said...

thanks. its a very informative post. really liked the same. thanks a lot.

Rudra said...

Hey! Jayant,

Your article is awesome.

Want to add something, now a days the Car manufacturers are trying to give a less emission Diesel engine.

Although the Diesel engines are costlier then their Petrol counterparts, but in the long run you can bet on it.

Anonymous said...

I drive a 1978 Mercedes 300D Diesel. It has lasted all these years, and still runs great. Here in the US, Diesel fuel costs a little more, but you can also make your own Bio-Diesel for very low cost.

If you get a Turbo-Diesel, you will NOT notice a difference. Good accelleration, power up hills... Without a turbo, it is a little slow on the accelleration or pick up, but very good considering how long the engine lasts.

In the long run, Diesel is the clear winner :)

Thanks for your time.


Unknown said...

this is great, useful and highly researched. I have used three disl vehicles ( VW golf 4, mercedes benz E200 and now hyndai H 1 or i800) for fuel economy and maintenance, petrol is out of the equation. God bless you

Eric, Ghana

surendra said...

yapp, jayant... Thanks for sharing ur valuable information with us. And bcos of that we knows the basic difference of these both engines.

Anonymous said...

Awesome little article. I'm thinking to go for a used diesel with low milage, prices are higher than petrol here in UK but when it comes to used cars I guess diesel would be more reliable and less fault sensitive than petrol with all the electrics etc. Any thoughts on used petrol v diesel cars would be much appreciated?

Anonymous said...

Thank you for this post.
I know nothing about mechanics - only money and I've been thinking hard about the fiesta diesel or petrol dilema.

I knew diesel had less performance than diesel but I get no joy from driving - it's a means to an end... An expensive means (after car prices, taxing, insurance and fuel). In the UK, fuels getting a bit out of hand and the price difference between petrol and diesel means less and less each week.

My annual mileage is average (6,000), but taxing a petrol Style+ costs £95 more than a diesel each year. MOT/servicing costs slightly more for diesel. Insurance quotes are coming out cheaper for diesel. But after many brain-straining maths calculations, even with my 6,000 miles, I'd spend £300 more a year on a petrol fiesta (based on urban driving using Ford's quoted mpg's for each model).

After reading what you have found, I'm convinced diesel will be the right choice (although I might check and see if the model I'm looking at uses 'glow plugs' incase I shoot myself in the foot and buy a car that won't work in winter.

Many thanks

hamza said...

Excellent essay about petrol engine vs diesel engines.

sand2006 said...

very good information

suja said...

hi,very much informative.

dirty diesel said...

Some good information here. One thing you did mention about the petrol was that it destroys lubrication and burns the engine that's why we keep topping up the oil to prevent such... What you missed about a diesel engine is that it can compress air too much causing knocking with this happening this can damage the engine. Also the diesel injectors are the most complex part of the engine if these go wrong these can drain any funds you have to get put right. Petrol and Diesel engines are a matter of opinion its what you prefer. Some petrol engines are now gaining the same mpg as diesels and yes diesel engines last longer but only if looked after and maintained same goes for a petrol I have an Audi A4 1.8T 143k on the clock that's diesel territory even though I am a dirty diesel fan because the torque in them is much much better remember...BHP sells cars Torque wins races!

property in mumbai said...

Wonderful article.
Looking forward to seek more of your fantastic post.

Anonymous said...

thanks for the info dude
buy something which runs on cng or hydrogen or water if ur in so much dilemma

kuldeepak said...

Hope this table will help, change figures as per actulas usage by you

Petrol Diesel
PRICE (INR) 700000.0 850000.0
KM/YR (KMS) 20000.0 20000.0
MILAGE 1500CC (KM/L) 12.0 16.0
LTS REQRD (L) 1666.7 1250.0
FUEL COST / YR (INR) 116666.7 68750.0
MAIN. COST /YR (INR) 10000.0 20000.0
OPERATIONAL COST (INR) 126666.7 88750.0

Anonymous said...

I have been driving for almost 50 years and have avoided deisel engines for most of that time thinking that petrol was better but four years ago I got a diesel and now will never go back to petrol
I have just got a new car and insisted on diesel
Where I live there are lots of steep hills and my friends and neighbours struggle in first gear wheras in my diesel car I can actually accelereate up hills.
That ability coupled with better fuel consumption and greater reliability makes the choice of diesel over petrop a "no brainer"


West said...

Good Article, but :

There are other things to consider too when it comes to diesel-petrol debate : drivetrain is second to engine in terms of maintenance cost and for a diesel with a manual gearbox (that is newer than 1997) drivetrain need change every 85k miles or so...whereas for petrol they last very long and only need the clutch disc changed according to driving style. So if you buy a diesel go for an automatic gearbox and if you buy petrol, well both manual and automatic are ok but the manual gives better mpg . People should know this too..

gopi kumar said...

Hi Jayanth,
Nice article. Ty. Pls let me know your present folks wagon is diesel or petrol?

pinky said...

hi jayanth thanx for d great info...can u plz clear my another doubt dat "is carburator used in all d petrol engines"....?

Unknown said...

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Petrol air compressor