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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