# What makes a Diesel engine more powerful than a Gasoline engine? **URL:** \ **Category:** Factual Questions\ **Created:** [November 7, 2003, 2:18pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887 "2003-11-07T14:18:54Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![Rayne\_Man](https://avatars.discourse-cdn.com/v4/letter/r/df705f/32.png) [@Rayne\_Man](https://boards.straightdope.com/u/Rayne_Man)\ **Post date:** [November 8, 2003, 4:17pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/21 "2003-11-08T16:17:38Z")
that last sentence should have read " … second-hand diesel cars etc. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [November 8, 2003, 5:42pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/22 "2003-11-08T17:42:23Z")
Did anyone mention they provide long-lasting reliability? I’ve noticed that most of the 15-20 year old Mercedes are diesel. Now, you don’t see many GM vehicles which had the Olds 5.7 diesel, cause…well…it was a converted gasoline engine, and was treated like a gas engine by the owners, mechanics, even the motor engineers who built the damn things in the first place! Reliability much improved with the introduction of the purpose-built 6.2 ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [November 8, 2003, 5:59pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/23 "2003-11-08T17:59:08Z")
> [@](#): > > \*Originally posted by Enola Straight \* > Now, you don’t see many GM vehicles which had the Olds 5.7 diesel, cause…well…it was a converted gasoline engine, and was treated like a gas engine by the owners, mechanics, even the motor engineers who built the damn things in the first place! And here I thought it was because nobody bought any of those cars in the first place! 😃 ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [November 8, 2003, 6:05pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/24 "2003-11-08T18:05:10Z")
> [@](#): > > \*Originally posted by Janx \* > **Sorry for the lack of attention here gang, the link that explains the difference between torque and power isn’t working for me, (I don’t want to join the group** Hit the “continue to message” button. ---
**Author:** ![keithnmick](https://avatars.discourse-cdn.com/v4/letter/k/90db22/32.png) [@keithnmick](https://boards.straightdope.com/u/keithnmick)\ **Post date:** [November 8, 2003, 6:07pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/25 "2003-11-08T18:07:10Z")
Europe has undergone a “diesel renaissance” in the last decade or so. Modern diesels are much quieter, cleaner and smoother with excellent fuel economy and reliability. They require newer, very low sulphur diesel to run which is not yet available in the US. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 8, 2003, 6:17pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/26 "2003-11-08T18:17:25Z")
Sorry for the hijack, but what’s the deal with biodiesel fuel? Would it be a direct replacement for petrodiesel? (i.e., would anyone notice if the diesel tanks at gas stations were filled with the former rather than the latter?) Or must modifications be made for a car to work with biodiesel? Does the U.S. have the agricultural capacity to produce enough biodiesel, so that if the majority of people switched to diesel-powered cars we could reduce or eliminate our dependence on foreign oil? ---
**Author:** ![aeropl](https://avatars.discourse-cdn.com/v4/letter/a/f05b48/32.png) [@aeropl](https://boards.straightdope.com/u/aeropl)\ **Post date:** [November 9, 2003, 1:33am UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/27 "2003-11-09T01:33:12Z")
Johnny L.A., this link answers some of your questions:[Biodiesel FAQ](http://www.biodiesel.org/resources/faqs/) From what I’ve read, there’s different grades of biodiesel. If you take used french fry oil and just filter it, it can run in a modified engine. Only when you process it more can it be used in a normal diesel engine. This is what is sold at some stations, but mixed in (usually at 20% biodiesel, 80% petroldiesel). This mix supposedly gives the best cost/benefit ratio. ---
**Author:** ![dqa](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dqa](https://boards.straightdope.com/u/dqa)\ **Post date:** [November 9, 2003, 5:25am UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/28 "2003-11-09T05:25:36Z")
Generally nobody in modern (last 5-10 years) cars would notice if fuel stations had biodiesel in their tanks rather than diesel - except in cold weather, when the higher gel point for biodiesel would become noticeable. I try to use biodiesel whenever I get the chance (no stations near where I live) simply because it acts as something of a solvent, helping to remove deposits built up from the crappy diesel fuel we have in the US. Also, it has a much higher cetane number - typically upper 50’s, compared with standard number 2 diesel typically about 42. This allows for more efficient combustion and thus smoother running. In spite of this, mileage takes a small hit - about 2 mpg in my experience. Reason for this is lower energy content in the biodiesel. Unprocessed fat - most commonly filtered **w** aste **v** egetable **o** il from restaurants - can also be run in diesel engines with minimal ill effects. However, it must be heated to reduce its viscosity. Usually a kit is installed in the trunk to heat the WVO, but I believe the engine must be started with regular diesel (or biodiesel) until the WVO gets warm enough. A couple of WVO converter kits [Greasel](http://www.greasel.com/) [Greasecar](http://www.greasecar.com/) Does the US have the agricultural capacity? From what I’ve read, no, and thus it will always be an alternative fuel. I should clarify: we _may_ have the physical capacity, but what’s allowing it to be cost-competitive (somewhat) at the moment is the fact that soybean oil (main source in the US) is essentially a byproduct of processing soybeans. Once demand for the oil outstrips demand for what are currently the main soy products, the soybean oil price will bear the brunt of additional farming and processing. On the plus side, the price of soy milk should drop significantly. Actually, what would happen first is a diversification into corn oil and other biodiesel sources. I am not certain how this compares to Europe, where rapeseed oil is the main source of biodiesel. ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [November 10, 2003, 3:27am UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/29 "2003-11-10T03:27:11Z")
> [@](#): > > Diesel engine isnt more powerful, but they develop more torque. That’s the basic answer to this thread. Diesel engines are less powerful. I believe that **Una Persson** (as Anthracite) posted a tech explanation which shot down the idea that there is or can ever be an engine which is “better for torque” as opposed to power. The upshot of it was that they are mathematically related in a formula that also includes RPM, in such a way that at a certain RPM the amount of torque in foot-pounds equals exactly the amount of power in horsepower. You’d get a different numerical crossover point if you were doing meter-kilograms and joules or whatever units you wish to use, but it would be a constant, and that constant would not be different for different engines. So saying such-and-such an engine is a torque engine rather and such-and-such other engine is a horsepower engine is like saying AC gives you more volts and DC gives you more amps. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [November 10, 2003, 4:22am UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/30 "2003-11-10T04:22:56Z")
It isn’t that diesels necessarily make more torque than gas engines- what they typically do is make much more torque at lower RPMs than a gasoline engine of similar displacement. For example(from the Ford F250 website): 6.0L diesel: ``` 325 HP @ 3300 RPM 560 ft/lb @ 2000 RPM ``` 6.8L gas: ``` 310 HP @ 4250 RPM 425 ft/lb @ 3250 RPM ``` So as you can see, the smaller displacement diesel makes LOTS more torque at 2/3 the RPM, and less HP at higher RPMs. For sports cars and aircraft, the gas engine would be better, while the diesel would be better for applications requiring a lot of torque at slower speeds. (i.e. tractor trailers, tanks, towing vehicles, etc…) ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 10, 2003, 11:22am UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/31 "2003-11-10T11:22:22Z")
> [@](#): > > \*Originally posted by bump \* > **So as you can see, the smaller displacement diesel makes LOTS more torque at 2/3 the RPM, and less HP at higher RPMs. For sports cars and aircraft, the gas engine would be better, while the diesel would be better for applications requiring a lot of torque at slower speeds.** If that’s all, it’s a simple matter of using a high gear ratio on a diesel engine to make the two behave exactly the same. I think you need to compare the actual shape of the torque curves, with the RPM normalized to make the peak (or something else?) equal. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [November 10, 2003, 1:51pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/32 "2003-11-10T13:51:37Z")
> [@](#): > > \*Originally posted by scr4 \* > \*\*If that’s all, it’s a simple matter of using a high gear ratio on a diesel engine to make the two behave exactly the same. I think you need to compare the actual shape of the torque curves, with the RPM normalized to make the peak (or something else?) equal. \*\* This was addressed before. Merely changing the overall gear ratio doesn’t compensate for a narrower power band. It takes more gear ratios spaced closer together. ---
**Author:** ![Janx](https://avatars.discourse-cdn.com/v4/letter/j/b9bd4f/32.png) [@Janx](https://boards.straightdope.com/u/Janx)\ **Post date:** [November 10, 2003, 2:23pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/33 "2003-11-10T14:23:06Z")
Now we gettin’ somewhere: \*\* bump \*\* wrote… > [@](#): > > It isn’t that diesels necessarily make more torque than gas engines- what they typically do is make much more torque at lower RPMs than a gasoline engine of similar displacement. If this is true (and I can’t say, or see where it wouldn’t be) then it makes sense to use this for off road where you may find yourself trying to stay ahead of the pack, yet still not breaking any speed records. For example, you go down a steep, hill at low RPM, through a creek, and then need to haul yourself up the other side, with out a whole lot of inertia on your side. Having geared down to pass the steep (downward) grade then pull the vehicle through the water, by the time you hit the up grade, your engine is at a low RPM, but you still need the torque (?) to pull yourself up with as much speed as possable. Is this right? ---
**Author:** ![KenGr](https://avatars.discourse-cdn.com/v4/letter/k/ecc23a/32.png) [@KenGr](https://boards.straightdope.com/u/KenGr)\ **Post date:** [November 10, 2003, 2:29pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/34 "2003-11-10T14:29:26Z")
> [@](#): > > \*Originally posted by AHunter3 \* > \*\*I believe that **Una Persson** (as Anthracite) posted a tech explanation which shot down the idea that there is or can ever be an engine which is “better for torque” as opposed to power. The upshot of it was that they are mathematically related in a formula that also includes RPM, in such a way that at a certain RPM the amount of torque in foot-pounds equals exactly the amount of power in horsepower. You’d get a different numerical crossover point if you were doing meter-kilograms and joules or whatever units you wish to use, but it would be a constant, and that constant would not be different for different engines. > > So saying such-and-such an engine is a torque engine rather and such-and-such other engine is a horsepower engine is like saying AC gives you more volts and DC gives you more amps. \*\* To understand torque, you’ve got to go a little farther back in the basic theory, to include BMEP (Brake Mean Effective Pressure). The BMEP is the average pressure forcing the piston down during the power cycle. As we all should know, torque is force times the distance it acts at (stroke). That’s why diesels, with very high compression ratios, have high BMEP and high torque. A gasoline engine with 18:1 compression would have about the same torque but, you can’t build a gasoline engine with 18:1 compression. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [November 10, 2003, 3:54pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/35 "2003-11-10T15:54:03Z")
> [@](#): > > \*Originally posted by KenGr \* > **To understand torque, you’ve got to go a little farther back in the basic theory, to include BMEP (Brake Mean Effective Pressure).** My post way back when addressed the criticality of BMEP. I wish Search worked so I could find it. Of course, nothing wrong with re-covering it as you did very well and succinctly. I have seen a laboratory gasoline engine with 14:1 compression, but it really was not very happy operating at that level. It sounded like hell when we had it on the dyno - not knocking, but just…very, very rough. (it was based on a Buick 231 V6) ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [November 10, 2003, 4:08pm UTC](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887/36 "2003-11-10T16:08:56Z")
> [@](#): > > \*Originally posted by Janx \* > \*\*Now we gettin’ somewhere: > > \*\* bump \*\* wrote… > If this is true (and I can’t say, or see where it wouldn’t be) then it makes sense to use this for off road where you may find yourself trying to stay ahead of the pack, yet still not breaking any speed records. > For example, you go down a steep, hill at low RPM, through a creek, and then need to haul yourself up the other side, with out a whole lot of inertia on your side. Having geared down to pass the steep (downward) grade then pull the vehicle through the water, by the time you hit the up grade, your engine is at a low RPM, but you still need the torque (?) to pull yourself up with as much speed as possable. > > Is this right? \*\* Sounds right to me, and the engines used in military vehicles seems to bear this out- the HMMWV has a 150hp(!) @ 3400 RPM 6.2L diesel that cranks out 290 ft/lb of torque at 1700 RPM. Sounds about ideal for what you describe- except that they run an automatic transmission as well. [Previous page](https://boards.straightdope.com/t/what-makes-a-diesel-engine-more-powerful-than-a-gasoline-engine/211887.md?page=1)