Is there a way to determine a theoretical point where an engine will operate at peak efficiency? Would it be something like the torque peak, or HP peak, or where the 2 cross, or maybe where they are closest, something like that?

As I was driving this weekend and playing with the overhead watching the mileage at different speeds, I began to wonder if there was a way to determine the point of greatest efficiency for an engine. I know there are tons of variables but discounting all the variables and looking at just the engine characteristics is there any way to determine where that engine would be most efficient? I'm sure if there is such a thing someone on this forum will know the answer.

I am not familiar with dynos but can you get fuel usage readings on the dyno that will tell you where the engine is most efficient?

 

Knowing where your engine is most efficient isn't going to help you much because of aerodynamic drag. The faster you go the more drag you have. That's why slowing down from 70 to 55 saves fuel. A Dyno won't tell you anything about your drag.

Your mileage display is probably giving you the best info.

Edwin

 

Generally the lower the boost the most efficiency. Most OTR drivers who are concerned about their mpgs drive by the boost gauge.

 

I ran engines on dyno's for a while at gm in the 70's and early 80's. We would run the engines at a specific load and rpm that would represent a certain speed in a given vehicle, and have several rpm/loads that we would run. We would make adjustments and modifications to the eng/fuel/timing/egr/int air pressure and temp, at all the road load conditions to monitor fuel consumption (bsfc), torque,HP, and emmissions to document which alterations/adjustments would make the eng put out what the engineer wanted it to do. Certain designs are more efficient than others, but emmissions controls changes some of that, and pressurizing the eng changes all that too. This can get very intense in explaination on design concepts and will change some from one eng to another and between years. The most influential thing that gives max economy is the speed you drive, how quick you get there, gear ratios, tires and inflation, the oils you use in the eng, trans and drive axles. Generally around 40-50 mph is where most vehicles get max economy. Can you drive 40-50 mph all the time??? I can't.

 

This explains why my "around town" mileage has been better than my "highway" milage. I can see 17.7 on the overhead while running around. more like 17.1-17.2 tops over long high speed distances.

 

I get my best mileage between 1800-2000 rpm's. I drive in 3rd lockup under 60mph. OD lugs the engine to bad and kills the mileage. I run 70mph, about 1850-1900 rpm, ~10 psi of boost, and get around 20 on straight highway. City mileage sucks for me. To much tire with to little gearing.

 

It seems like this could be a complicated question becouse the first thing that comes to mind is emissioins that detune both gas and diesel for less polution and that brings in a idea I had if detuning burns more fuel, than is there really less polution. but to get closer to finding a answer to your question , in a gasser a vacume gauge is the simplest way I've used and I would guess that the boost gauge,[ I haven't put gauges on my truck yet] in a compressed intake would sever the same answer. those are about fuel to air ratios only ,then you have to bring in relitivity , enviromental , air resistance and how fast to accelarate and can you maintain an efficient speed , relative to laws and other drivers.

 

as far as the thermodynamic diesel cycle, the higher the cylinder pressure and the more heat the higher the efficiency. more boost & heat = more efficiency.

that does not necessarily mean better fuel mileage. it takes less fuel to spin 1800 RPMs vs. 2500 RPMs, even though 2500 RPMs may be the most efficient RPM of the motor. *just picking numbers out of the sky* also, more fuel = more heat. efficiency for a given RPM goes up when a load is applied because it causes more boost, but at the expense of more fuel being used and the more fuel is also more heat to increase the efficiency some more. kinda sounds like a catch 22.

diesels are great because your foot determines how much HP/TQ you are using. which brings up the question, how much self discipline do you have?


Pat

 

I'm not sure if dynos can do this, but I would suspect that there's some fancy ones that would allow you to add a load in order to simulate the additional load of aerodynamic drag.

That would let you test for best economy, wouldn't it?

 

Joel: "That would let you test for best economy, wouldn't it?"

With your ideal dyno, you could test for best economy, but what if the testing showed that you should drive at 18 MPH for best economy? (extreme example)

What you need to do is TUNE for best economy. Pick a speed and a load that represent your situation, for instance, 10,000 lb trailer and 65 MPH. Set this into your ideal (mythological?) dyno with the appropriate wind resistance load dialed in, and program the dyno to simulate a variety of roads and hills (variable power needed). Then you can start changing parts such as turbos, injector pumps, intercoolers, injectors, drive ratios, exhaust systems, while you do multiple runs to measure the results of your various combinations. Are you wealthy? Am I nuts? This process will cost more than driving an untuned truck.

In a way, DC and Cummins have done this for us, with the dubious help of the EPA. You just have to choose the right vehicle. For light loads and groceries, use a 1500 or a Toyota. For construction work, use a 2500. For your 5er, use a 3500. They are designed and tuned for an average of what DC thinks people will use them for. Of course, if your usage is not average or you are not satisfied with the tuning, you are welcome to start changing parts...

I am in the process of tuning my truck to haul my camper and boat cross country at 62 MPH. I make one change before each trip so I can measure the results. Reliability is important. Different drivers get different results on my truck. It's a slow process.

 

There's a diesel engine energy efficiency map available in this EPA pubication. I suspect this is for a small TDI-type diesel engine, but typically peak efficiency should be at the peak torque RPM and at relatively high load.

 

Peak efficiency under a load would be at peak torque rpm, I believe.

 

Under full load conditions peak torque takes place at the RPM when peak BMEP (brake mean effective pressure) occurs. This is generally also the "sweet spot" on the brake specific fuel consumption (grams of fuel/BHP-hour) efficiency map.

At light loads, all bets are off.....

Rusty

 

I can't tell you guys how much I'm impressed with all the replys. Except with CCM591's. And its not that I think his reply is wrong or that I am not impressed with his, but I think his compared to the others is just a bit vague.

The other guys hit the point. Just becuase an engine's best efficiency (meaning that's where the engine will burn fuel most completely) is at say 2,400 rpm, doesn't mean that the truck/vehicle will achieve its highest fuel economy at that rpm. I have ranted on this point until I was blue in the face and seemed to never make a dent to anyone. But basically every reply I agree with 100%. Just read the above again, because my opinion is the same as all above. Even CCM's, just that I think his post seemed ... unfinished.

- JyRO

 

I remember there were BSFC curves in the TDR magazine about 3 or 4 years ago for just this topic. And Rusty is right, the efficiency curves using BSFC would be most accurate at Full load conditions. However you can interpolate that the scavanging and pumping losses of the engine at that RPM are the lowest and peak efficiency, even at lower loads should be at that RPM. That is pretty well proven over the years as the different engine models have their own "sweet spots" for mileage.

If I can get those curves found and scanned I will post em..

J-eh