Diesel power vs Gas power

[depasseg]

I'm starting a new thread here, but it relates to this post in the simplicity forum where someone was asking about getting a diesel vs gas engine. (http://www.mytractorforum.com/showpost.php?p=54351&postcount=38)

One of the replies is something that I've heard before but don't understand:
A DIesel of the same HP as a Gas engine has more power than the gas one.

I thought HP was a measure of power. Can anyone explain what that really means? Is it a difference in torque curves? Is it the way power is measured? I'd like to understand what the real difference in power is between the 2 types of engines.

Thanks

[Greg]

Without getting technical, it's been said that Power = Torque, and Speed = Horse Power. Simplistic answer, for what it's worth. Can anyone expand on this? Correct it?

Uncle Greg

[mbielick]

FYI.
Diesels operate on a different torque curve. Gasoline explodes on ignition, resulting in immediate expansion, and short term pulsing of power. Diesel fuel is VERY slow burning compared to gasoline. This means the power generated by a diesel is developed over a longer portion of the power stroke. What this means is that, for a given HP rating, a diesel will generate equivalent torque at a much lower RPM. Gasoline engines can show similar differences too. Compare a high reving 4 cyl DOHC engine to a big V-8 of the same HP. The high reving 4 will almost always have lower torque output.

The net of this is the diesel will be able to continue through the power stroke where a gasoline engine will stall.

[cadurning]

In the generator business horsepower at a given rpm is horsepower no matter the source. We deal in engines that run at a constant 1800 rpm so this may not apply to a tractor. But in our experience it to obtain like horse power the gasoline/nat gas engine must have more cubic inches to do the same job as a diesel. As an example; we use a 502 (8.2L) Chevy engine that is turbocharged & after cooled to do the same job as a 6.6L naturally aspirated diesel could do, both at 1800 rpm.

Now in my sprinter that same 6.6L diesel would be a turd. My smaller 350 (5.7L) clearly shines in that application.

So it all comes down to what you want to accomplish. There are other trade offs also. For the money in a lawn mower a gas engine can't be beat for light weight and simplicity. For real work get a diesel.

[depasseg]

Thanks mbielick - what you've written makes sense. But let's suppose for the sake of example that I have a tractor with a gas engine and the exact same tractor with a diesel engine and both have the same HP rating and I run them at full throttle, am I going to be able to notice a difference in what I can push, scoop, drag, or lift between the two? I would think not, but I don't know. I would also imagine that if what you said is true, then the diesel wouldn't be required to use as much fuel/rpms as the gas to generate the same power. But I figure that the engine manufacturers take this into consideration. Also, would the HP have anything to do with the capability of the hydraulics (I would think that the tractor manufacturer would design the pump to meet the expected output of the engine), or just the ability of the tractor to move under heavier loading or going uphill, for example?

On my legacy xl the max throttle is ~3500 rpm. Does someone have a reading for the diesel legacy xl of what the max rpm is to compare?

[mbielick]

At full throttle, you would not see a difference between gas and diesel until you begin to load the engine down, and lose RPM. The diesel will resist this bogging down better than the gas unit of equal HP. The diesel on my Legacy runs at 3200 - 3400 RPM at full throttle. Diesel fuel contains more BTU's per gallon than gasoline, which accounts for some of the increased efficiency of the diesel.
You are absoluetly correct in that the hydraulics is what moves the machine, and that it doesn't matter what HP your engine is if the hydraulics can't keep up. The only time you would see a difference in operation would be when the hydraulics are under heavy load, either from climbing hills, or doing heavy auxilary work off the PTO. The gasoline version and diesel version would perform the same as long as the engine doesn't bog down. If the load bogs the engine down, the diesel will perform better.
Cadurnings comments about the generator application are very true. 1800 RPM is very low speed for a gasoline engine, but close to optimal RPM for a diesel. Therefore, the diesel will outperform the gas unit due to it's ability to maintain greater torque at lower RPM's. Thus, you would need greater displacement for the gas unit to keep up.

Mark

[bontai Joe]

I just happen to have a 2005 Dodge truck catalog here at work. They state the 5.7 liter Hemi (gas) engine developes a peak HP of around 345 at 5400 RPM and the 5.9 liter diesel developes a peak HP of 325 at 2900 RPM. Similar dispalcement engines capable of similar HP but the torgue curves are VERY different, with the diesel producing 600 ft. pounds over most of the RPM range and the hemi producing over 300 foot pounds over most of it's RPM range, according to the graph on the corner of page 13. The diesel gives you more torque in this example by almost a factor of 2.

[JDFANATIC]

The best example I can give is my 2210 which is 23 hp can at idle engage and run the 62C MMM without bogging down. Go and try that with a X485 or 585 gas power. I can't imagine a condition where the diesel wouldn't put more power to the task than a gas engine. Also, when it comes to maintainance, life, and resale, the diesel has it all over gas.

Cheers

JDFANATIC
JD2210

[Mickey]

To answer the original question accurately one needs to know how "power" is defined and used when talking about things such as these.

First, torque is a force around a rotating shaft and can be measured. Second, HP can NOT be measured as it is a mathematical calculation. It is a defined amount of work over time.

There are a lot of differences in the otto and diesel combustion cycles that I will not get into. Typ diesels are an under square design i.e. smaller bore than stroke, and gas engines are closer to or are over sq designs. Longer strokes make for more torque for a given pressure on the piston. Typ diesels run at about 50% of the rpm of a gas engine. For equal HP, the diesel has to have a lot more torque than an equivalent gas engine because of the speed difference at rated speed. The torque curve (torque Vs speed) of a diesel is likely to be flatter than a gas engine.

In application the diesel's higher torque, at lower speeds, you are likely going to see a better response to varying loads that one typ sees in use.

If the load, work, is constant and no variations in speed, both engine designs will do the same amount of work IF the actual power is the same at the speed the engines are running.

As for fuel consumption, diesel fuel has a higher energy content than gas and the thermal efficiency of a diesel is also higher than the gas engine. Combine both of these facts and a diesel will consume less fuel for the same about of work.

[depasseg]

Thanks for all the replies. There's a ton of good info!
I called my dealer, and asked if I could return/upgrade. He offered to let me pay the difference between what I bought the liquid cooled for and the price of the diesel ($700). I guess it seems worth it. I have a question though regarding how power gets to the wheels. I have a hydrostatic transmission. Does this affect anything? I could imagine that with a manual transmission, you could control the gear you were using, and not apply the clutch (allowing the rpm's to drop, but still having power/torque). Will a similar thing happen with the HST, or will it stop driving the wheels before the engine will?

I hope this makes sense. It's similar to my question before of hydraulics - if the pump is only capable of so much, a bigger engine/PTO isn't going to give the hydraulic implement any more capability. Is a HST the same limiting factor?

Thanks!

[depasseg]

Does anyone know if a hydrostatic transmission will take advantage of a diesel? I mean, in a manual transimission, the operator can choose to leave the transmission engaged until the engine stalls. Will a hydrostatic transmission do the same? Or will it give up before the engine stalls?

Thanks,
Greg

[Keweenaw4310]

Does anyone know if a hydrostatic transmission will take advantage of a diesel? I mean, in a manual transimission, the operator can choose to leave the transmission engaged until the engine stalls. Will a hydrostatic transmission do the same? Or will it give up before the engine stalls?

It depends on the hydrostatic transmission design. Without adding any control logic to it, you could bog down and stall a hydro drive machine.

On my JD4310 with EHydro, there is a load match control which will cut out the drive as much as necessary depending on the load to the system. With load match on, stalling is much less likely.

The EHydro also has control for how quickly the transmission responds to pedal controls of forward and stop. You can go from full speed forward to stepping on the reverse pedal and the machine will stop at the selected rate of deceleration then switch directions and accellerate at the selected rate of acceleration