Oil & Fuel Gas vs Diesel PTO HP

[tinytractors]

This may sound strange..., but is 24 PTO Hp the same for gas and diesel. My thoughts are that it is... but the amount of torque available at that HP differs between a diesel and a gas engine.... and that is what sustains the work being done (more with diesel)... or is this just a case of apples/oranges.

Thanks, Pete.

 

[DmansPadge]

You are correct. The horsepower might be the same but diesels have more torque than gasoline engines. Torque is turning force. Both gas and diesels have appropriate applications. But if I'm using a tractor... I want a DIESEL!

 

[Egon]

It may all depend torque/rpm and how the engine is designed.

Egon

 

[Dargo]

Yes, the torque makes a huge difference in available power. For example, when I was going to buy a zero turn radius mower, I looked at several 25hp gas models and a 21hp diesel model. I had each brought out to my property to try. In the same height thick grass, the 21hp diesel model would mow considerably better than the higher 25hp gas mowers would. In fact, there was a huge difference. After doing the actual side by side comparison in tall thick grass, it really proved to me that for mowers and tractors, torque is what you need. Not horse power. Based on actual results, I'd say that you would need a 30hp gas engine to do the work of a 21hp diesel engine. That's a huge difference! PTO output would be similar.

 

[ericinmich]

I’m going to agree with most/all of what has been said, but want to add my thoughts for a very specific situation.

If both a gas and diesel engine running the same RPM (ex. PTO speed 2600) and have 25 HP available, then both are putting out (at that speed) the same torque. After all, HP is just torque times RPM.

So, if you had a 25 HP load on the PTO, and that load never when up or down, then the engines would feel like the same power.

But, in the real world loads are always changing, and many times we are not running the engine at PTO speed. That is where the differences are easily seen. A diesel has a flatter (or even negative sloping) torque curve, and a gas engine has a positive sloping torque curve.

The bottom line is that in real world use, where you run the tractors engine at 25% to 100% PTO speed depending on the task, a diesel is much better suited for the job.

- Eric

 

[WilliamBos]

............but if you look at dyno sheets of a gas vs a diesel the torque will peak sooner and last for a longer rpm. ADVANTAGE DIESEL.

just my 2 cents.

Will

 

[hudr]

I have operated three different fuel types of tractors..gasoline, LP Gas and Diesel. The diesels just don't seem to "lug down" as much when put under load. They may hammer and smoke, but they keep right on pulling. I have had all three fuel types in the hay fields with similar HP ratings and the diesels just seem to get more done on less fuel and with less strain. I still use an old JD 720 LP (2 cylinder) from time to time, but if I really need a workhorse, I climb on one of the diesel tractors.
Just what I have noticed from my seat.

 

[Hypernix]

hudr,

Just curious, what is LP Gas? /forums/images/graemlins/blush.gif

Hypernix

 

[KiotiJohn]

</font><font color="blue" class="small">( Just curious, what is LP Gas? )</font>
Liquid propane. In the tank it's liquid, but comes out as a gas. Used for stoves, furnaces etc. John

 

[ericinmich]

Will - I think you are saying the same thing as I did... just different words (see my comments are neg. slope on tq curve).

 

[slowzuki]

Two other things too, diesels then to have more rotating mass including bigger flywheel. This means when small surges of power demand as smoothed out better.

The other thing is a diesel is speed governed directly via the injection pump while a gas engine on a tractor is speed governed via the throttle plate. In any case a diesel with good torque rise can respond to loads quicker and avoid being dragged down.

Some diesels can temporarily exceed its rated output when doing this. Gas engines can't usually exceed their rated output. Only some much fuel they can draw in no matter what.

 

[MMM]

I tend to think a lot of it has to do with volatility of gas. Higher cyl pressures from heavy loads on crankshaft are more favorable to diesel engine, where gas under same pressure will pre ignite, heat up comb chamber, and make predetonation worse. hence the knock of a gasser under heavy load. My observations from owning both fuel type tractors.

 

[Charlie_in_TX]

</font><font color="blue" class="small">( I’m going to agree with most/all of what has been said, but want to add my thoughts for a very specific situation.

If both a gas and diesel engine running the same RPM (ex. PTO speed 2600) and have 25 HP available, then both are putting out (at that speed) the same torque. After all, HP is just torque times RPM.

So, if you had a 25 HP load on the PTO, and that load never when up or down, then the engines would feel like the same power.

But, in the real world loads are always changing, and many times we are not running the engine at PTO speed. That is where the differences are easily seen. A diesel has a flatter (or even negative sloping) torque curve, and a gas engine has a positive sloping torque curve.

The bottom line is that in real world use, where you run the tractors engine at 25% to 100% PTO speed depending on the task, a diesel is much better suited for the job.

- Eric
)</font>
(finger pointing to nose) You hit it on the head. If the PTO is turning 540rpm and has 25hp there will be the same amount of torque. HP = torque * speed * a constant (that depends on units for torque and speed). The advantage of the diesels is when they are loaded and slow down. Say the load slows the PTO by 10% (540 to 486 rpm). The motor connected will also slow 10%. A diesel motor that goes from 3000rpm to 2700rpm will likely see a torque increase and keep the hp near the same. A gas motor will loose torque and far more hp than the diesel.

 

[Dachshund]

Well - even though the diesel has the advantage under load - I'll stick with a gasser. Used to have a diesel - got tired of fixing it. It's just my personnal preference, but I like the gassers. Diesel exhaust also messes with my siniuses, so I stay away from it!