This is a rather long explanation that I orignally posted at type-r.org, Irish will love this!!! Hopefully this will enlighten you garethj
Petrol Vs Diesel
In reality it is the torque that reaches the drivewheels that dictates both acceleration AND max speed. This figure is comprised of engine torque and gearing.
Higher engine torque = higher torque at the wheels = higher acceleration.
Lower gearing = higher torque at the wheels = higher acceleration.
Remember that TORQUE is the only engine output a driver feels. Power is a kind of esoteric measure that is simply calculated from Torque and Engine Speed. If we are talking BHP and lbs.ft then the equation is:-
Power= (Torque x rpm)/5252
But the power on most cars is rated at the flywheel and not the wheels so the powerloss through various transmissions is:
FWD 15%
RWD 20%
4WD 25%
Power dictates how much advantage you can make of gearing
Ive seen a few analogies before, but the best one Ive have ever seen is the analogy of the human body as we all now the basics of how we work don't we? So I shall use this analogy of Bill and Ben on their bikes.
Torque is what your legs generate. If you press harder on the pedals youll accelerate harder. You use the gears on your bike to multiply the torque your legs generate. In 1st gear the bike wheel may only rotate once for every 5 times you spin the pedals. In this gear you can zip up a hill (or accelerate really quickly), as this has thegearing has the effect of multiplying your leg power by 5.
If you maintain a steady force on the pedals, the bike accelerates at a steady rate. The torque you are generating stays constant, but the faster you spin your legs the more power you muster. Eventually you discover that you run out of of steam and cannot peadle any faster, and you cant really get any force into your leg strokes. This is like your engine going upto and beyond its peak power point.
Higher gears allow you to move along quickly whilst your legs go round at a comfortable pace. Nice one. Try to pull away from a standstill in the same gear however and (without enormous multiplication of your leg strength) itll take you significantly longer to get up to speed.
Bill and Ben are out two exaples of engine. Bill has very strong legs. But struggles to pedal very fast. So whilst he can put alot of force into each reveloution, he can only manage slow rotations. This is the good old TDi
Ben conversely has very athletic and supple legs - . He has hoever very flexible and fast moving ankle muscles (think VTEC) that helps him to peddle very fast. Although his legs lack power he can pedal very fast.
We line them up on the drag strip and the light goes green and Away!! Bill storms into the lead, his rear tyre barely able to contain the thrust created by those strong legs. But he soon cannot pedle any faster, and hes quickly forced to change up into second, then third
and look at this! Ben is staging a comeback, still in first with his legs going round alot faster than Bills. Whilst Bill is producing major leg thrust, hes currently using 3rd gear, which only doubles the torque his legs generate before it reaches the rear wheel. Bill is still in first, so his supple legs continues to be subject to 5-times multiplication
and hes reeling Bill in!
Its a similar story when they are up to maximum speed. By enormous coincidence they appear to have an identical flat-out pace. Bill is pumping his top gear firmly and steadily ~ wind resistance and friction conspire against him he can go no faster. Ben is right up there with him, but hes still in 3rd his legs pedaling faster than Bill.
Back to the world of cars. You should now be able to understand why TDIs are often quoted as having massive in-gear thrust, whilst its acknowledged that they are not quite as good when it comes to both 0-60 and top speed although they are catching up ie BMW 330D or VAG 150TDi. Most TDIs barely get to 60 in third, so in addition to requiring an extra gear change over most petrol models, they pass the magic 60 in 3rd ~ which is by definition higher geared than 2nd, so torque is multiplied to a lesser extent before it gets the chance to arrive at the drivewheel and do the business. This gearbox multiplication is the real important factor and why two engines of the same power output can seem faster.
As diesels operate using compression ignition they dont require spark plugs. This omission unfortunately makes advancing the spark somewhat difficult the main cause of their high rpm torque defecit.
Because ignition cannot be adjusted like a petrol car they cannot rev to such a high level as a petrol engine.
A VTEC engine with 140lbs.ft max torque CANNOT punch as hard as a TDI with 200lbs.ft. In a VTEC however you will choose to accelerate with a lower gear than the TD, so the torque it does have is multiplied to a greater extent before it reaches the wheels. A TDI might feel impressive in 4th~ certainly more impressive than a VTEC in 4th, but the VTEC driver would undoubtedly pop it into 3rd (or even 2nd) to make better progress. The TDI has no such option. So high RPM means that we can stay in a lower gear for longer!!
As a rule - lower gearing makes better use of what torque you have available, but leads to unfashionably (and often uncomfortably) high-rev cruising.
This gearbox multiplication is the real important factor and why two engines of the same power output can seem faster. It is also why some people say this engine feels more tourqey even though engine A has the same power/torque as engine B.
Another note needs to be made of chipping, most engine re-maps will increase the power but it is the torque that they concentrate on.
This ia a simplified version of somthing that helped me understand the realtionship between Petrol and Diesel. It shows why Petrol still has the edge but Diesel is nearly there.
As the increase in Power for a diesel comes from an increase in fuel/air density which comes from the injection pressure currently at 1600 for a common rail system and 2000 for a PDi system (VAG). The increased pressure gives more torque at any set engine speed for a like for like engine capacity. This leads to smaller more powerful engines able to match Petrol via the Gearbox/torque ratio.
