Fuel rate setting and heat rejection of engine ?

[schiker]

Ok, I think I know more than just the fundamentals of how an engine and a diesel works but let me eat a little humble pie and ask a few questions. I am going to put it here cause I am a 6.5er and it relates to a naturally aspirated IDI engine I am working with.

Bare with me...(I am going to generalize some numbers).
Given an engine has a heat rejection rating at an output will the heat rejection be less at lesser load with the same "fuel" position?

Say a 50 hp naturally aspirated engine is rated for 40 hp continuous load at WOT being 100% loaded. And its rated at 40,000 BTU's/hr heat rejection at this 100% load at this 40 hp.

And its a simple idi mechanical injected engine. So WOT or any throttle position is a set fuel rate of injection. And you set a specific throttle position on an unloaded engine. Say 2600-2700 or very near this governed and rated rpm above. Will the engine require the same 40,000 btu/hr radiator capacity without load or a lesser load?

Or since its really just pumping air at no load will heat rejection be much less say 5,000 btu/hr again at no load.

Again set engine RPM to near WOT the above rated HP output of engine via "throttle position" with no load on engine...
Then if you load a 10 hp load to crank will it require say 15-20,000 btu/hr of cooling.
Load it to 20 hp it will require 20-25,000 btu/hr cooling.
Load it to 30 hp it will require 25-35,000 btu/hr cooling.
Load it to 40 hp it will require the 40,000 btu/hr cooling.

I think that is correct and why a turbo requires fuel and load to build boost. It takes fuel and load to make heat.

OR does it being a diesel engine the fuel rate is set and if you set "fuel pedal" stationary it injects the same amount of fuel per injection event and thus requires cooling according to fuel position. ie if you set WOT or very near its 40 hp worth of fuel being injected and thus 40 hp worth of fuel requries 40,000 btu/hr of heat rejection regardless of load?

 

[buddy]

Are you trying to put on a smaller radiator? Trying to decrease cooling ability? at idle on a hot day your no power situation may require more cooling than your highest power while moving at high speeds.

 

[schiker]

Yes, this is not a truck its a piece of stationary equipment and the size of the radiator cost money and we have to price it competitively.

We'll heat test it and make sure it is adequate for specs once built up. Just looking at options for now.

The engine is oversized for the amount of horsepower required due to marketing and perception.

If another company offers 100 hp and you have to quote a spec bid against it with a 75 hp it looks bad. You generally have to match specs and price competitively. Next question you could ask is why is the engine over powered and there is no good answer just look how powerful trucks have gotten and how many actually use even 50% of the power.

I am pretty sure my understanding is correct then waffled a bit and thought I would double check.

 

[Missy Good Wench]

For any given amount of fuel geing burnt there is a given amount of BTU's being produced.

So, the engine really does not care if you are feeding it 20mm3 of fuel at no load or 20mm3 fuel with it pulling its guts out.

With a no load situation the given fuel rate may well place the engine hard on the governor.

The only real way is to figure your max fuel rate under worst case conditions with maximum load.

Low speed with minimal air movement can see a heat build up but this is due to a lower air flow accross the radiator and less coolant flow through the system.

Do your figures under worst case scenario and then give your little creation a margin of safety that includes something to allow for degredation of the cooling system over time.

Far too often (As in many diesel pickups from detroit) the systems are designed right on the edge of the envelope and with a small amount of degradation due to normal wear and tear the system becomes less than capable of coping.

If this a piece of equipment that must be competative, make it reliable too and your good name will shine through.

In my experience, original cost is only one part of the equation. The end user will call smack on you until you die if your product lets them down when they need it most.

Pay me now or pay me later.

The exact amount of over engineering is up to you, but, if you dont do it you will regret it.

Hope this helps.

MGW

 

[schiker]

For any given amount of fuel geing burnt there is a given amount of BTU's being produced.

So, the engine really does not care if you are feeding it 20mm3 of fuel at no load or 20mm3 fuel with it pulling its guts out.

MGW

Ahhh but does it?

With my truck sitting stationary I can't really build any appreciable boost no matter if I romp on the pedal any which way. But if I hold my foot steady going down the interstate boost reacts with load (hills).

From just a bit of heat testing I have done it appears the loading has a direct correlation to how hot the engine runs.

I was hoping to confirm the idea that a radiator sized for 75% output of a 100 hp engine ie 75hp load could in fact provide acceptable cooling for a 50-60 hp load. With safety factors and calculations to figure its ok for this on a 120F degree day.

 

[knh208]

What you will find is that max fuel rate likely cannot be sustained in a no load situation, because the governor will cut fuel rate to prevent exceeding rated RPM. If the governor did not cut fuel rate, then the engine would be placed in a "runaway" situation, where the RPM increases until one of a few things happen:
1) The valves float
2) Friction and energy requirements within the engine become high enough that it takes the maximum fuel rate to turn it that fast
3) Something fails catastrophically (either rods come out the side of the block, or the unburnt fuel creates EGT's so high that pistons, valves, turbos all melt into shapeless blobs)

Stationary engines are often controlled by a variable governor rather than directly with fuel rate, such that setting a "throttle position" will actually set an RPM, and the governor will vary fuel rate to maintain that RPM.

That's my understanding. I hope it is helpful.

 

[schiker]

Stationary engines are often controlled by a variable governor rather than directly with fuel rate, such that setting a "throttle position" will actually set an RPM, and the governor will vary fuel rate to maintain that RPM.

That's my understanding. I hope it is helpful.

Thanks, Thats what I have been thinking it has to work like that some kind of way. But haven't heard anyone confirm it that way.

I have asked the engine distributor to estimate the heat rejection at 75% load. To see if its appreciably different.

The distributor has repeatedly said they are application driven for component size. And if we wanted to design for MAX output worse case scenerio it would up the price appreciably and might be overkill for actual application.

Robyn I appreciate your comments and agree but then again have to justify the cost and present options to management. I have been asked specifically to look at cost. And will heat test the actual build before release. This is just to validate a somewhat prototype build plan and see if the distributor has properly sized the radiator. Or if I need to change it (actually by the numbers the radiator should be a bit bigger than distributor has sized but not sure we want to put on a bigger more expensive radiator if we don't have to). Its just to have a plan to start from at this point.

 

[buddy]

With a stationary motor I agree it should work like you say. Youll have to look at power curve to see at what RPM you make your desired HP though. The more linear that is the more your original presumption holds true.

 

[schiker]

Good point referencing the power curve. I don't expect it to be completely linear and why I put a range of numbers it was just easier to start the conversation with that kind of estimation.

I had also thought the heat rejection might be somewhat load dependant and not just fuel rate dependant because.... Seems with the Diesel cycle and over fueled or less load than fuel required it just naturally runs up rpm and the injection event gets sped up and cut off wether it wants to or not. It finds its equilibrium of resistance to load vs fuelrate sorta naturally. But when its a heavy load it feels like it really burns the fuel and is working differently and more grunt pull even though you are not really pushing more fuel. But I am not sure if that is a turbo sensation or Diesel in general.

And even though its the fuel doing the work and not a compressor. Its still based on PV=MRT. And without the resistance to piston movement the fuel expands more easily. And was kinda thinking the easier it expands (pushing the piston down the less heat soak and expanding gas is cooler than when it has to really push the piston down with heavy resistance. Again thinking why driving down the road without load it just doesn't build the same boost.

Yes heat rejection is based on fuel burn and fuel has BTU/gal. But its not as easy as that because of ineffeciency of cycle and depends on how much heat goes out the tail pipe vs what is actually converted I think.

Thanks again it has helped me wrap my head around it a little better.