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Given two identical cars except for the engines (diesel vs. plain gas) why does the diesel have more torque? What inherently makes a diesel engine have more torque? Sorry for the ig'nant post.
 

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Good question. One thing that I have noticed with my truck is that because it runs at such low RPMs it is rated at a very low horsepower compared to its torque. 245hp and 505ftlbs. I love it, I can take my foot off the clutch and pull my 22' sail boat up the boat ramp without ever touching the accellerator.:clap:
 

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My best guess is it's a combination of compression and displacement. Basically, diesels are pushing huge cyllinders at low RPM, thus producing lots of torque. Since HP is based upon torque x RPM, they will always have a proportionally low HP figure.

Diesels ain't so low on the HP these days, though. You can find many many big diesel pickups pushing 800 hp and 1600 lb-ft and doing 11-second 1/4 miles !!! :eek: (according to 4wheeler mag june 2004)
 

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I believe compression is the biggest factor. IIRC diesels typically run compression in excess of 20:1. They are also direct injection motors which allows for more precise fuel injection.

Check out the MB gas 3.2L V6 and 3.2L I6 diesel. The diesel makes 20 less hp but 167 lb-ft more torque. The E320CDI is actually faster than the regular E320.
 

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Torque will most likely be a result of a longer stroke. Diesel fuel has slightly more (25%?) energy per unit of measurement, but I don't think that alone could explain it. Back to the longer stroke, with that, piston speeds at a given rpm will be higher, thus the engine cannot withstand higher rpms, and so the rest of the engine - camshaft profiles, for example - are optimized for low rpm operation. WHY diesels are targeted for low rpms I'm uncertain, but I suspect it is traditionally due to limitations in the manner of fuel injection, and difficulty making that work at higher rpms?
 

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Probably over-simplified, but here's what I have read about diesel engines:

Instead of using spark plugs to ignite a compressed air/fuel mixture, diesel engines compress air only and rely on the heat generated by the compressed air to ignite fuel.

To generate the heat necessary, higher compression ratios are used. While gas engines range from 8:1 to 12:1, diesels range from 14:1 to as high as 25:1.

Heavier components are needed to withstand the higher compression ratios. Although the heavier components will lower peak RPM, they alow a longer stroke and higher turbo boost levels.

==> Diesel engines favor torque vs. horsepower.


I expect diesel engines to make a comeback, especially when the ultra-low-sulfur "clean diesel" EPA mandate takes effect in 2006, and especially if gas prices continue to rise.

Until then, as Tom mentioned, there are products like the Mercedes-Benz E320 CDI. Although it has a little less HP than a regular E320 (201 vs. 221), it has more torque than the 5L V8 in the E500 (369 vs. 339). Add to that 37 MPG on the highway, which is unheard of in a heavy luxury sedan. However, it's not available in states with strict emissions requirements like CA and NY.

As Matt mentioned, diesel is higher in energy density than gas (~18% higher). That plus the higher efficiency from higher compression ratios equals better mileage. And since diesel requires less refining, it's usually cheaper than gas. It's no wonder that diesel is so popular in Europe.
 

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Again an oversimpliflication: it's the stroke that creates torque. The longer the stroke the more torque you'll get. Torque is a measure of angular energy versus HP which is a measure of absolute energy. The longer the stoke the bigger the crankshaft has to be. The bigger the crankshaft is the more leverage the crank has and the more torque there is. With more compression the longer the stroke has to be and the bigger the crank has to be.

There are more factors that affect the Torque vs HP amounts (fuel used, back pressure, etc) but the above is good starter on understanding it.

Also desiel burns slower than gas so it make since to slow down the engine to make it more efficent. With any fuel the more that gets used the more "effficent" it is. So if it takes long to burn more fuel then you just slow down the RPM's and give the desiel more time to burn more completely.

And add in losses for rotating mass, friction, reciprocating mass and it all adds up to low RPM's being more efficient.
 

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Lumn8r said:
Wasn't someone getting astronomical mileage with a diesel powered Elise?
Diesel packs more energy-per-liter than petrol (about +20%), so it's not surprising that it would get better mileage with the same engine efficiency. But diesel engines are usually more efficient at extracting the energy from the fuel, so you get another bonus there.
 

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I'm no engine man, but I may be one when I grow up, finally.

There are two books that I would like to recommend that, if you want, give a good explaination of engines

1. Classic Racing Engines by Karl Ludvigsen gives accounts of 50 racing engines from 1913-1992. The first engines were markedly "undersquare" where the stroke was up to double the bore. After WWII, engine became square where stroke equalled bore. Now with the high revving engines, they are oversquare with large bores that are close to doubling the short crank throws.

The 2ZZ-GE comes in at 80x85 (BxS, R&T 11/03)

2. Romance of the Engines by Takashi Suzuki. This book chronicles internal combustion engines from the first that used gunpowder to power water pumps at Versailles, to radials, diesel, Turbine, to Hydrogen. Very technical, very complete.
 
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