automotive engine performance 2

[berkshire]

I need to ask a question about engine performance.
Most automotive engines have a power curve shown by the manufacturer. However in practice most automobiles do not load the engine that highly, in that max power is only used for acceleration or hill climbing.
I am looking for a petrol (gasoline) motor to drive a generator, the input requirements are 150 to 200 horsepower continuous for up to 6 hours at a constant speed. By how much would I have to derate a motor to have it be reliable? That is not overheat nor get excess bearing wear. For example would I need 250 hp to get 200? Estimated use rpm would be 3600.
B.E.

 

[patprimmer]

Diesel engines used in long haul trucks have a much higher duty cycle that passenger cars.

Marine applications have a somewhat higher duty cycle than passenger cars.

If you compare engine life in hours of a passenger car vs a marine version of the engine you should get a good feel for the comparison.

If you want to just buy an engine with good durability at continuous high load operation, buy a diesel motor made for a prime mover.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[icrman]

I think most auto manufactures proof test their engines at
max rated HP for X amount of hours. To run full rated HP as continuous you would need a cooling system to handle it.
And most likely some other type of exhaust system.
Since there have been many automotive engines converted to aviation and marine purposes, which would require continuous HP production. That atests to the ability of those engines to handle that kinda duty cycle. And yes they are derated, as in aviation full mas power is only used during take off. And then you throttle back and maintain X amount of rpms.

 

[bentwings1]

If you use a gas motor for this you will need a tank truck to supply gasoline. Use a diesel. 5.9 Cummins will still be running into the next melinium. You won't even have to derate it.

 

[GregLocock]

In a couple of companies I've worked for the first serious test of a new gasoline engine design is 100 hours at full power. Once you get through that you know you have a reasonably durable engine - it will have many other durability problems, but the fundamentals are there.

Some engines pass first time. Others take more than a year from the first try to actually doing it. I've been tangentially involved with both situations, trust me, a first time pass is far less hassle!



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

heheeh... a medium duty diesel mfr might call that a "break-in."

 

[berkshire]

So Greg,
Is that 100hrs at 100% on a dyno?
I agree with some of the other suggesters that a diesel would be better if the engine I was asking about was actually running on petrol. The other suggestion of checking marine ratings has some merit.
However the engine will be running on Methane gas from a slurry digester in a commercial pig house, this unit will house about 20,000 pigs.The Idea is to extract the available energy before the slurry is dumped in a farmers field.This idea also is not new. I did not mention it before because I did not want a dozen responses on how to do a gas conversion. My main concern is reliability with the smallest displacement possible and the longest life.
I would like to get 5 to 6000hrs before a rebuild.
So would I run at 50% 75% 80%? there has got to be a law of diminishing returns in here somewhere.
B.E.

 

[ivymike]

5 hours before a rebuild should be easy. 5000 hours before a rebuild with a load factor over 50% means you'll need something FAR more robust than an automotive gasoline engine (5000 hours at full load would be akin to buying a car and driving 130mph for 650,000mi).

As long as you only want power, durability, and low cost, it should be easy enough to find a product that does it all, right? If you can't find one, pick one (or more) of those three that you'd be okay with compromising on, and start shopping again.

 

[GregLocock]

yes, that's at 100%. I entirely agree, a diesel is far more appropriate for that sort of maintenance cycle, but there may well be a tradeoff here, a larger SI engine run at low speed may be a sufficiently cost effective solution.

Why do you want a small displacement?





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

Let's see 3,600 Rpm at 365 Ft Lb is 250 Hp.

That might be marginal for a small block V8, but a big block will do it more easily, especially if speed control is critical. Find something with a torque peak around 3,600 Rpm maybe ? As others have said 3,600 continuous Rpm is working an engine pretty hard.

A diesel running at 1,800 Rpm would be my choice for a reliable long life stationary application.

 

[berkshire]

GregLocock (Automotive
The amount of methane generated by anerobic bacteria is temperature dependent. When the weather is cool the output falls off, unless efforts are made to route waste heat through the digester. However the 3600 rpm figure is not cast in stone. It was a suggested number put out by the manufacturer of the alternator for direct drive. If I belt drive the system I can select for max torque. Then I get into another set of issues regarding bearing loads on the rear main bearing;one I was trying to avoid.
The 5000 hour figure was wishful thinking. the best I have seen on automotive engines has been around 2,000.
B.E.

 

[berkshire]

Ok guys,
I have just discovered that I am trying to re-invent the wheel.
I found a unit by Generac with a 4.3 litre V6 already set up for natural gas that will work. Best part is, all the engineering is done.Thanks for your efforts.
B.E.