Why do turbo-diesel engines have such high torque?

[BeEfCaKe]

Why is it that turbo diesel engines have so much more torque than hp? For gasoline engines, usually the hp and torque numbers are very similar, but since I've been considering buying a TDI Jetta recently, I was studying them, and noticed hp being only 100, but torque as "high" as 177.

Also, with only 100hp at the flywheel, how much hp can I expect to get to the ground? Will this be very slow?

[383PhoenixAm]

Turbo diesel engines are mostly intended for trucks and other heavy things. That's mostly the reason for the torque. They need it to start moving. I have no idea why they would put that in a car. Blame the germans.

[SaabJohan]

Typically 10 to 20% of the power is lost in the transmission.

The reason why diesels give so much torque is just because they operate on such low engine speeds. Compare the BMEP's with any turbocharged gasoline engine and they will pretty much be similiar.

The reason diesels are used is because of several reasons, the torque isn't one of them. Diesels are more fuel efficient (less CO2 emissions), they last longer and their fuel is safer, less flammable.

[Kven]

well the diesels run shit-high compression, which explains the torque.

[sracing]

Wrong, wrong and wrong.

The compression ratio is NOT directly an issue of a diesels torque.

Torque (at low RPM) IS why a diesel is primarilly used. (trucks, industrial equipment etc.) In passengers cars it is primarilly used for less pollutants, some economy, and some fads.

A diesel engine is significanty different than a gas engine. For one thing the fuel (not unlike kerosene) burns through most all of the power stroke. (Giving much more torque) Due to this slow burn, the typical diesel can't reach the RPM's that a gas engine does. The typical diesel also has a higher stroke to bore ratio, thus making it more torque (all other things being equal). At 5252 rpm ALL recipricating engines produce the same HP and torque. (Of course some can't reach 5252 )

Consider the steam engine.. It produces it's maximum torque at ZERO Rpm.

The diesel (when used as designed) is more efficient and cleaner.

Diesels can be produced that make similar HP and fitted with gearboxes that make the same cruising speeds with better economy and less pollutants than a gas engine. The drawback would always be it's acceleration. Due to the fuel type (and other things) it winds up slow. It's inertial HP looks very bad (on an inertai dyno for example), but it's steady state HP (on a eddy current dyno for example) looks just as good as it's comparable gas engine.

Also, the first poster asked about how much HP he would put to the ground with a 100hp diesel. About the same as you would with a 100hp gas engine (steady state HP). (With a stick, figure a 10% - 13% loss, with an automatic figure 15-20.

However at the diesel it would be somewhat slower in acceleration all other things being equal. The manufactures rate them with net brake hp. They do not use inertial measurements. (acceleration.) (See above)

Jim
SR Racing

[Kven]

ummm diesels do run high compression...have you seen the pistons they use? i know its not the main factor that creates torque but high compression>low compression, power wise.

[civickiller]

diesels dont use spark plugs, they combust by compressing the gas and air mixture.

[kcap122]

Diesel engines basically run on knock (sparkless detonation of low-octane fuel). Diesel fuel also has alot less energy itself than unleaded gas, so that contributes alot to hp loss.

Also, you'll notice the TDI cant rev that high. the reason is, like someone said already, that diesel also takes a longer time to burn. So, if you bring them above a certain speed, you get almost no combustion, but you willl get a whole lot of unburned fuel running through the exhaust valves.

bottom line: If they didnt make torque, they wouldnt sell.

[BeEfCaKe]

Originally Posted by kcap122

Diesel fuel also has alot less energy itself than unleaded gas, so that contributes alot to hp loss.

That's not true, diesel fuel actually has higher energy density than gasoline.

[BeEfCaKe]

Thanks for all the quick replies. I looked up some 0-60 times for the TDI(1.9L) and its somewhere in the 12s, which really is a bit slow. I think I will have to consider other alternatives to save gas.

[Alastor187]

Originally Posted by kcap122

Also, you'll notice the TDI cant rev that high. the reason is, like someone said already, that diesel also takes a longer time to burn. So, if you bring them above a certain speed, you get almost no combustion, but you willl get a whole lot of unburned fuel running through the exhaust valves.

I personally have never heard of this extended length of combusting time for diesel fuel. As I understood the limitation of increased engine speed was that the amount of time to inject and atomize the fuel decreased. Whereas with a SI engine this is not a problem since the air-fuel mixture is mixed prior to entering the cylinder allowing more time for atomization and mixing producing a nice heterogeneous mixture.

[bjdm151]

Diesel does have a higher energy than gas, 130,000 btu vs about 112,000 for typical gasoline.
In a diesel, only air is in the piston on the compression stroke(20:1 compression), closer to the top, fuel is injected at a very high pressure and rapidly atomizes. diesel doesn't have an octane number but a cetane number, which measures the amount of lag between injection of the fuel and the begining of the actual combustion. Diesel does have a longer burn time because it has longer molecular chains than gasoline.

[bjdm151]

And just so you all know the word of jim schings is law....usually

[sracing]

Originally Posted by bjdm151

And just so you all know the word of jim schings is law....usually

Is that you, BJ? You no longer have to suck up to me. (until you come back to work next year). (and don't say "kiddies" or BUZZZ" <GG>)

Jim

[CBFryman]

Many of the replys on here make me laugh. Diesel engines Make far more torque for reasons sracing said. Bore to stroke ratios, longer burning time of diesel and the fact that the diesel beignes to ignite with in a few degres either way of TDC. Diesels, however, are not as able as gasoline engines to reach the same RPMs as gasoline engines; and since horse power is the ammount od work which can be done in an aloted time speed, along with power, is key. Horse power is directly related to torque. high torque at low RPM's (ie. diesel) is good for large/heavy loads when trying to start off or climb a steep hill, but this limits maximum speed. low(er) torque at high RPM's (ie 2 stroke gasoline) is good for small vehicles with high top speeds (for their size). Diesels do not have a carburator or a throttle plate. engine speed an power is determined on how much fule is injected. Diesels run lean most of hte time. air fule mixture becomes more and more closer to the perfect mix as engine speed increases since more fule is injected into the engine. when a diesel does run fat it usually releases black smoke. diesles do, however, run extremely high compression. 12:1 is a high compression in 4 strok gasoline terms, but most diesels wouldnt run or would run like @$$ if they had a 12:1 compression. average compression for diesels is in ups of 1:20 give or take a few points. Boost for car engines is anywhere from 6psi (> 0.5 atmospheres) to 23psi (~1.5 atmospheres). there are extreme cases where gasoline engines run in ups of 30-40PSI of boost but the compression ratio is very low in gasoline terms or they arent runing pure gasoline. there are additives such as lead that can be added to gasoline to get a higher octane rating and there are gasoline alternatives such as Ethnyl and Methane (methane is extremely volitile and is usually mixed with Ethnyl or gasoline). Diesels, on the other hand, can run 30psi easy. i have heard of diesles running in ups of 150psi of boost (>10 atmospheres). basicly if yo ucan get the oxygen into the cylender and enough fule to use that oxygen a diesel will make masivly large ammounts of power. want to increase torque which will increase HP? Cram more boost in the engine and inject more fule. your turbo at its limits? nitros oxide can be used. want to save money on disel? propane injection is a very commonly used method and is very often used with nitros oxide. since propane is in gas form almost immeaditly after leaving its LP holding tank it does not need to vaporize and has a very high octane rating so, unless running massive ammounts of boost already, you wont cause misfireing.