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Old 20th September 2009, 13:23   #1
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Default What the difference between a Diesel and Petrol engine??

this is the mother of all my automotive doubts. sure they run on different fuels, but what is the difference between petrols and diesels?? for instance, petrol engines generally have more horsepower, but diesals have more torque. why is this?? diesels also require less maintenence unlike petrols. why?? also can someone explain the working of the diesel engine?? the dont run on spark plugs do they?? help me out guys!!

Last edited by bivin : 20th September 2009 at 13:26.
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Old 20th September 2009, 13:56   #2
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OK I'll start with some beginner's stuff over here. This is what I remembered my from school time .

The petrol engine requires a spark plug to ignite the fuel-air mixture in the cylinder. But, the diesel engine doesn't require a spark plug since the compression of the fuel-air mixture in the cylinder is so high that it generates enough temperature to ignite it. Speaking more precisely in Diesel engine the air is compressed before injecting it into the cylinder. The compression of air increases its temperature making it ready for ignition after mixing with the fuel. Of course the final compression occurs in the cylinder that ignites this mixture. As far as I think this is "Turbo charging" of the diesel engine. I believe older diesel engines or the non-turbo ones don't have the air compressed before entering the combustion chamber.

You can see an animated version of the Diesel engine here and the Petrol Engine here.

You can expect a more personalized and detailed explanation of these two engines in the forthcoming posts.
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Old 20th September 2009, 14:04   #3
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So why do diesels get better torque and less power? This is a very useful thread btw for a comprehensive petrol diesel analysis
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Old 20th September 2009, 14:09   #4
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And what properties of petrol and diesel make people to design completely different engines?

Also with kerosene both engine can run with little bit of modification.
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Old 20th September 2009, 14:12   #5
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A Google search with what's the difference between petrol and diesel engines turns up some 793,000 answers, most of which are pretty much the same. IMO, this thread will add nothing new to this pool of information.
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Old 20th September 2009, 14:22   #6
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From thermodynamics, both are IC engines and one follow diesel cycle and other has otto cycle. I am sure if you hunt on t-bhp and google around you will find more tech diffs, but, some fundamental differences in layman terms:


1) In Petrol a mixture of petrol + air is compressed by piston, and at the highest point (Top Dead Center TDC) of piston, spark is generated- causing a controlled explosion (and sudden expansion of gasses) of fuel + air mixture, forcing the piston (+connecting rod + crank shaft) to move down- generating power.

2)In a diesel only air is compressed first, which increases its temperature enough to ignite fine droplets of diesel, which is forced into the combustion chamber later. Rest all are very similar to petrol engine.

3) In petrol engine CR (Compression ratio) i.e. Initial volume of fuel mixture/final volume is about 8-12. I.e. 1200CC 4 cyl engine, each cylinder having 300CC, the initial volume of air + fuel would be 300C, and, if its CR is 10, it will be compressed to 30 CC before igniting.

4) In diesel engines, CR would be around 15-20, i.e. 300cc will be compressed to about ~15cc.

Since, diesel engine has to compress more, its parts (piston, connecting rod, crank shaft, flywheel etc) are heavier (and stronger) than petrol engine.

Since, petrol engine parts are lighter, they can spin faster than diesel counterparts. Hence, a petrol engine RPMs are higher than diesels.

As diesel engine parts (flywheel/crankshaft) are heavier, they can "twist" more- more torque than petrol engines.

Since diesel parts are bulkier (=stronger) and moves slower- they also wear slower than petrol parts.

Petrol is a hydrocarbon (CH) formula of smaller molecules of carbon and hydrogen (C4-12) while diesel is a mixture of larger hydrocarbons (C8-20), meaning, less energy is required to break petrol molecules while more energy is required to break diesel molecules. As a corollary, petrol releases less energy than same quantity of diesel when burnt at identical conditions.

Yes, since CR of diesel engine is more and petrol is more volatile, you can't put petrol in a diesel engine- it will pre-ignite petrol causing knocking, however, reverse is less harmful (but, does not run engine).

One more diff, contrary to popular belief, Diesel engines are more efficient and less polluting than Petrol. The energy efficiency of petrol engines are about 20% while diesels are about 30-35% or so, meaning, only 20% of burnt fuel is converted to useful mechanical energy to drive petrol car , while about 30% or so of the burnt diesel energy is gainfully used to drive diesel cars. This manifests in a different way too- you will notice petrol engines warm up faster (loss more heat) than diesel engines.

Hope this was simple (and correct) enough to explain the main differences.

Last edited by bj96 : 20th September 2009 at 14:31. Reason: added note on efficiency
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Old 20th September 2009, 15:04   #7
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Quote:
Originally Posted by McLaren Rulez View Post
So why do diesels get better torque and less power? This is a very useful thread btw for a comprehensive petrol diesel analysis

Quote:
Originally Posted by bj96 View Post
Since, diesel engine has to compress more, its parts (piston, connecting rod, crank shaft, flywheel etc) are heavier (and stronger) than petrol engine.

Since, petrol engine parts are lighter, they can spin faster than diesel counterparts. Hence, a petrol engine RPMs are higher than diesels.
Refer my reply above.

Torque = twist
Stronger heavier the twisting arm, stronger is the twisting force. Diesel wins.

Power = movement (or work) / minute

Since lighter parts (in petrol vs diesel) can move faster i.e. spin/min more => more power. Petrol wins.


Quote:
Originally Posted by anujmishra View Post
And what properties of petrol and diesel make people to design completely different engines?

Also with kerosene both engine can run with little bit of modification.
It is the stoichiometry and heat characteristics of these items. Refer my reply below.

Quote:
Originally Posted by bj96 View Post
Petrol is a hydrocarbon (CH) formula of smaller molecules of carbon and hydrogen (C4-12) while diesel is a mixture of larger hydrocarbons (C8-20), meaning, less energy is required to break petrol molecules while more energy is required to break diesel molecules. As a corollary, petrol releases less energy than same quantity of diesel when burnt at identical conditions.
Since all these are hydrocarbons- they will burn, given enough air (O2) and heat (ignition).

Carbon composition:

Petrol = C4 - C12
Diesel = C8-C20
Kerosoene = C6 - C16

So you see all these fuels are having interleaving composition, so you could tweak your engine to burn all of them and generate mechanical energy, but it will be sub-optimal. Hence, engines are tweaked to burn certain fuel completely and extract maximum energy, rather than burn anything (but incompletely). In pterol it is tweaked to burn C4-C12, in diesel it is C8-C20 and so on. If you don not do that, the efficiency will reduce, and fuel will remain unburnt, increase pollution, catalytic converter fouling, knocking etc.

Btw a form of kerosene is also known as ATF (Aviation Turbine Fuel) and it is used to fly airplanes and they also use ICE

-BJ

Last edited by bj96 : 20th September 2009 at 15:06.
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Old 20th September 2009, 16:29   #8
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People havce already explained the difference fantastically.

Just referring to my post in another thread in a diff context but give some insight into what is being described here. Follow the link:

http://www.team-bhp.com/forum/indian...ml#post1492241
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Old 20th September 2009, 17:00   #9
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Good post @oss.
Just want to add some perspective on importance of torque. You see an average tractor has about 30-40bhp only while some two wheelers made here now have about 25bhp. But a two wheeler can't plough fields or haul a trailor!. The crux is a 2 wheeler spins to about 8000 rpm with a tiny torque while a tractor w/ a mammoth torque spins to about 2000 rpm only. Torque is real while power is more marketing
-bj
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Old 27th September 2009, 13:23   #10
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Quote:
Originally Posted by leonerd View Post
OK I'll start with some beginner's stuff over here. This is what I remembered my from school time .
Guess you weren't paying much attention in school. Was it the girls ??

Quote:
Originally Posted by leonerd View Post
Speaking more precisely in Diesel engine the air is compressed before injecting it into the cylinder. The compression of air increases its temperature making it ready for ignition after mixing with the fuel. Of course the final compression occurs in the cylinder that ignites this mixture. As far as I think this is "Turbo charging" of the diesel engine. I believe older diesel engines or the non-turbo ones don't have the air compressed before entering the combustion chamber.
Saying this because you are confusing compression ignition and turbocharging, a lay person reading will get the wrong message. You can turbocharge a petrol engine as well, and compressing air prior is not a necessity - You rightly say this at the end. So your first sentence in the above quoted post is not worded right, though I can understand what you are trying to say.

Air is never injected, the diesel fuel is - can be direct injection(straight into cylinder) or indirect(into the inlet tract - older technology).

Also, in a diesel engine, speed is governed by directly controlling the amount of diesel injected, compare to the older technology of using a throttle(inside the carb) for air-fuel mixture in petrol engines.
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Old 27th September 2009, 19:50   #11
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Small correction, indirect injection in a petrol engine involves injecting fuel into the air intake, in an IDI diesel, there is a separate prechamber where the fuel is injected to begin combustion.
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Old 28th September 2009, 15:36   #12
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Quote:
Originally Posted by teknophobia View Post
Small correction, indirect injection in a petrol engine involves injecting fuel into the air intake, in an IDI diesel, there is a separate prechamber where the fuel is injected to begin combustion.
Thanks for the correction.
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Old 28th September 2009, 16:04   #13
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Quote:
Originally Posted by leonerd View Post
Speaking more precisely in Diesel engine the air is compressed before injecting it into the cylinder. The compression of air increases its temperature making it ready for ignition after mixing with the fuel. Of course the final compression occurs in the cylinder that ignites this mixture. As far as I think this is "Turbo charging" of the diesel engine. I believe older diesel engines or the non-turbo ones don't have the air compressed before entering the combustion chamber.
The Turbo or Supercharger or any other external means used is to push more air into the engine. In a normally aspirated engine the maximum amount or air that can be sucked into the engine is governed by the atmospheric pressure. Under load conditions the diesel fuel injected into the engine will not burn completely as the air available is not enough for the combustion of the amount of fuel injected.hence wastage of fuel. Black smoke from the exhaust is the result of unburnt fuel. In a Turbo charged or Supercharged engine more air is forced into the engine and the fuel burns completely. Hence you will not see any black smoke from the exhaust under any load condition normally. May not apply to most of our trucks ( reason:- a truck with 5 Ton load capacity may be carrying 7 ton or more load ).
Supercharger uses some of the engine power to run where as Turbocharger uses the waste exhaust gas power ( Free power ) to run the Turbo.

Last edited by Rehaan : 28th September 2009 at 22:55. Reason: Quote fixed.
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Old 29th September 2009, 05:46   #14
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ATF ( Aviation Turbo Fuel) & ATK ( Aviation Turbo Kerosene ) are one and the same. They are ordinary kerosene but quality is strictly controlled.

Also petrol engines will run on diesel with less power and too much smoke. ( have seen a Villiers petrol Irrigation pump set running on diesel creating a white cloud in the paddy field because some idiot bought "fuel for the pump set" ( Diesel instead of petrol) and poured into the kerosene compartment of the fuel tank. These engines have 2 fuel compartments, one for petrol needed for starting and the second for kerosene for running.

Last edited by sudharma : 29th September 2009 at 05:47. Reason: remove quotes
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Old 29th September 2009, 12:47   #15
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Quote:
Originally Posted by sudharma View Post
ATF ( Aviation Turbo Fuel) & ATK ( Aviation Turbo Kerosene ) are one and the same. They are ordinary kerosene but quality is strictly controlled.
Minor point- the ordinary kerosene sold (via PDS) in India is "adulterated" with a little dye to give it a bluish tinge and make it (~Rs20/lit) easily identifiable so that they do not end up being smuggled into the petrol (~Rs50/lit) bunks causing (more) dents on the pocket of (exploited) working class like us.

Both PDS and subsidy are funded by you and me, but, politicians market them as their gift to the aam aadmi...

-BJ
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