Engine Power Vs Temperature and Humidity

[Danandersen]

Hi Everyone,

Im running a Wauskehsa VGF H24. I want to know the effects temperature and humidity has on its performance, I know the general stuff;
high temp and high humidity an engine usually looses power. But how much can be expected on an engine of that caliper?

Today it is 96 degrees with the relative humidity humidity of 85% ( this is Florida )
My engine is rated at 614 Amps - 410 Kw But today it maxed out at 525 Amps ~ 360Kw. Is this to be expected or should I look into mechanical malfunction?

Any input would be appriciated

 

[gruntguru]

All you need is a psychrometric chart or calculator Link.

At the 96F, 85% RH the specific volume is 14.7 cu.ft/lb dry air. Not sure what the std conditions for rating your engine are, but at 68F 20% RH the specific volume is 13.36 so the engine output would be potentially 13.36/14.7 x 100 = 90.8% under the hotter, more humid conditions. This translates to 410 x 0.908 = 373 hp.

Low barometric pressure will also reduce power.

je suis charlie

 

[Lou Scannon]

Whether and how much the engine output derates with ambient conditions depends a lot on the engine controls. The more modern engines that I'm familiar with do not derate with the relatively normal ambient conditions that you're reporting, considering that your elevation in Florida can't be very high, hence your ambient pressure should be close to sea level.
I suggest you check with your Waukesha dealer for information about scheduled power derate for your specific application, and go from there.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Panther140]

Is it maintaining normal levels of boost?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[Danandersen]

Gruntguru:
What are STD conditions? Do you mean STP? Running at 90.8% is very close to a rough calculation I did, added with the barometric pressure.

Hemi: Yes, I am at sea level. Modern Engine. my engine is from 2001.? I am aware that the VHPs tends to vary in Hz and power during high humidity, seemingly the VGFs does too.

Panther140. Yes, I have same boost. about 14-15 psi.

 

[Tmoose]

Is this the same engine that got the "defective" new wastegate a few months back?

http://www.eng-tips.com/viewthread.cfm?qid=386528

If not, do both ( all ?) the engines respond the same losing power to the weather?

 

[Danandersen]

Tmoose,

Yes, same engine.

I think I mentioned it in the other thread, but I put the old wastegate back in, and I have since then run the generator at higher loads with no problems. I have had it at 575 amps since then.

The difference now between the other situation is that I do have the correct boost, (about 15 psi) If it is not temp and humidity, and my wastegate is seemingly working I dont know what else could cause this to happen. timing is okay, all sparks are working, there are no restriction in air flow, Carb is set.

Any other suggestions?

The other engine is not running at high enough load to really see if it loosing the same amount of power I think.

 

[Danandersen]

An added datum to that, My intake manifold temperature is 140F while the design is at max 100. So I am running hotter then Im supposed to.

I got a datum somewhere that for every 5 degree increase in temp you loos about 2% power? could this be so?

 

[Tmoose]

This suggests they expect intercooler (inlet?) water temp no higher than 130 F.

http://www.md.cl/descargas/especialidades/energia/vgf/vgf24gl-gld.pdf

Not sure how that could be expected to lower the inlet manifold temp to 100F.

 

[Danandersen]

Tmoose.

I am not sure this would be intercooler inlet.

The nameplate on both my engines says 100F going in.

 

[gruntguru]

Charge air temperature is important. An increase from 100F to 140F would reduce power by about 7%.

je suis charlie

 

[Danandersen]

Gruntguru.

7 percent is quite a bit. How did you arrive to that number? I've been looking around but haven't been able to get any literature on the subject.

7% is also only about half of the power loss I seem to be experiencing. any other ideas?


Thanks

 

[Lou Scannon]

It really comes down to whether the performance recipe combined with the control system is capable of controlling charge mass flow, with margin above nominal conditions, or not. If yes, there will be a manufacturer's derate table vs. ambient temperature and pressure, and a zone of pressure and temperature where full power is delivered. Compensating for humidity is more complicated, but can be done also, with appropriate sensors and algorithms. All this provided there is margin in the first place. More sophisticated systems modulate not only charge mass flow, but also air/fuel ratio and even spark timing, in order to maintain a more or less constant heat release profile for any given operating point, within a defined envelope of ambient conditions. Engines without these types of controls operate in a manner that I would call "free-floating", and will deliver variable power at WOT in accordance with well known physics of gas properties and combustion.
I have no idea how the Waukesha engines are configured, but the non-Waukesha engines I'm familiar with have plenty of margin to maintain rated power and (as applicable, torque, below rated speed), within defined limits of ambient temperature and barometric pressure. All while operating within applicable emissions regulations.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

7% is the reduction in air density when heated from 100F to 140F. High humidity will further reduce the oxygen content of that air.

Of course all this assumes a constant AFR. For example if the engine is lean-burn and mixture control is a simple carburettor, less oxygen simply results in a less-lean mixture and power does not suffer at all.

je suis charlie

 

[Danandersen]

Hemi: I have contacted WPI regargind the a "derating" table, and waiting for an answer. I am certain the engine does not compensate for humidity - there are no sensors like that. I would say it is a "free float" as you call it. Im sure the engine also can operate within a certain limit.
I guess, what Im trying to find out, is how much I have to surpass that limit before it makes an effect and how big the effect is going to be at a given point outside those run parameters. Im not expecting a huge loss in power if the engine only runs at 10F higher ambient then the nameplate calls for. So, if the engine now is running at 15% less capacity as described earlier, would this then be caused by the high humidity and the high ambient and intake manifold temps? are those factors sufficient for such powerloss?

GruntGuru:
Oh okay, I understand the 7 % . I do not quite understand how the engine would not loose power if you richen it towards Stoich. Does it then just run Hotter, or?


I could learn a lot from you guys!! I really appreciate your input.
Thanks

 

[gruntguru]

As long as you are leaner than Stoich you have more oxygen than needed to burn all the fuel. It is only when you reduce the oxygen beyond stoich (rich side) you are no longer able to burn all the fuel and power must fall.

The above refers to a situation where fuel flow is held constant and the oxygen flow is varied. In this scenario, best power will coincide with best brake efficiency - typically at a mixture which is 10% - 20% excess air.

je suis charlie

 

[Danandersen]

Gruntguru:
Alright, that makes sense. So if I check my O2 and I read 7.8% at pretty much all loads except from idle or no load would that, then, mean that I have varying fuel and oxygen?

 

[gruntguru]

I was referring to the reduction in oxygen available per unit volume, when the intake air gets hotter or more humid. There are three variables you have mentioned so far: ambient temp, humidity and charge air temp.
a) If the fuel system is designed to maintain a constant fuel/oxygen ratio (exhaust sensor feedback), the fuel will be reduced in line with the reduced oxygen availability and each of the three variables will cause the power will suffer.
b) If the system adds fuel in proportion to the volume of air consumed (carburettor with no O2 compensation) the power will suffer from high charge air temp. High ambient temp or humidity will richen the AFR but that won't affect your output much.

When I mentioned Std conditions in an earlier post I meant the ambient conditions under which Waukesha rated the engine.

je suis charlie

 

[Lou Scannon]

As I said, only the engine manufacturer can tell you for sure what to expect for a specific engine model, depending on how it does or doesn't compensate for changes in ambient conditions.
But assuming no compensation, calculators such as this can give you an idea of what happens due to ambient conditions, when the physical throttle valve is completely wide open. However, that actual results with a specific engine model depend on many things, such as whether or not it has an electronically controlled throttle (and in that case, what is the control strategy relative to the plant input, e.g. "dumb" or "smart"), whether or not it is turbocharged (and in that case whether or boost is controlled and in that case if it is operating within or outside of its control range), whether air/fuel metering is done with venturi, mechanical fuel valve, or mass flow basis; and what open loop or closed loop compensations might exist, such as humidity compensation, lambda closed loop, or "gas quality closed loop" (a Woodward Inc. concept).
I have not vetted this particular calculator, and due to the caveats above I doubt it is applicable for your particular engine, or indeed, for any but a narrowly defined range of engine & control system combinations.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz