How can I Derate my Engine Efficiency from 86%?

30-odd cc/min comes to something like 1 us gal in ten minutes - at idle?!

Sorry - just re-did my sums - I was talking crap (and too late to edit ).

It's more like 1 us gal / 2 hrs.

There is no way your engine will be 30% efficient at idle (net output is actually zero), but considering the power it is using to run itself as your output it still won't anywhere near that.

The reason is that the efficiency of an IC engine is dependent on the pressure ratio, ie. between the peak cycle pressure and the lowest pressure. This is why raising your compression ratio increases both power and economy (not much else does). But when an engine is idling, the theoretical compression goes out the window because the throttle valve lowers it. You would be lucky if your effective compression ratio was 4.5:1, with the attendant low efficiency that goes with it.

I am sure you could find a curve for efficiency versus compression ratio on the web to see what I mean.

The throttle valve effectively detunes your engine until its output just meets the engines running needs.

ok so at idle there is no output but what about friction and compressive losses can you put a figure on that eg like a compressor?

My engine is approx 58% better on fuel when driven under city conditions I pretty sure you cant just add 30% to 58% to end up with 88%. Can I take a 58% increase on 30% which is 47.4% or do you have to work out the difference in energy consumed?

Here's a tip from an old engineer. When you do new complex calculations that involve unit conversions do them twice with different sequences and compare the answers. If you are going to publish your results do the calcs at least 3 times and try to get a friend to check them for you.

Ed Weldon

I am not an engineer just a backyard mechanic/radio and TV tech and dont know anybody that can help me thats why I have turned to the internet for help.

I used to have a normal engine for comparsion but I had to leave it behind when I split up with my girlfriend some 4.5 years ago, the police would not help me retrive it without a court order I asked them twice.

I guess Im after someone to measure the fuel consumption of there car I guess there are not too many 4 cylinder 2000 cc engines with a carby left on the road now, I think if you filled your tank and idled you EFI engine for 2 hours then filled it up again you may have a reasonable figure to report, but its finding someone who is willing to do that.

what ever you did is not reversible then?

I'm trying to understand what you want to learn, so I'll make a list:

fuel consumption at idle

fuel consumption under various conditions

you need this info for an unmodified & modified version of the same motor

ideally both dyno & long term road tests

did I leave anything out?

Smokin -- I commend you for this experiment you are doing and your interest in putting some real numbers on what you are observing. You're a bit burdened here by not having an engineering education or years of experience with the subject of engine performance. That's OK. We all had to start somewhere.

One of the real problems you face is that there isn't an easy way available to the average shade tree mechanic to measure horsepower output of a car engine . If you are going to try and work with measurements that require that data you need a bit of a financial war chest. I could make some suggestions; but I'm pretty sure you would have to pass on those items on the menu. So we'll leave that stuff for later.

Fuel consumption at idle is of less meaning because there are so many immeasurable variables that affect the power draw from the engine. First off, at idle, the engine is putting out very little power. That means the temperatures in the combustion process in the cylinder are way down. Thus the theoretical thermodynamic maximum efficiency is low to begin with. Next understand that the amount of fuel used at idle is small and unnoticed by the average driver when they observe mileage when most of the time the engine is up at 1500 +/-rpm and pulling a load of 25-50 hp rather than the 5-10 hp at idle. When you set the carb for ordinary driving you set the RPM with a meter or if you're good at it just by "ear". You really have no idea when you do that what the actual throttle position is at. The net result of all this is that idle power setting and the resultant power consumption can vary a huge amount and the differences from one car to another will be hard to notice. And hard to measure.

"If I can measure it I can make it?" I've forgotten which pioneer of our Industrial Revolution said that. But it applies today. It's one of the foundation blocks of engineering methods. (Are they worth anything? Ask the Romans.) So, Smokin, what can you measure well enough to be believable? 1. Amount of fuel. Gas pumps are pretty accurate. 2. So are mileposts on highways and your speedometer/odometer. As well your digital watch. 3. Your brain is pretty good at keeping track of events and even better if you write down what you've seen and measured.

What's important to you, the automobile user? Well, idling performance may be important if you are stuck in the sand in some remote piece of the Arizona desert and your car air conditioner is the only thing keeping the summer sun from killing you. But that is not a concern for most of us who tend to plan ahead somewhat.

What does concern us is mechanical efficiency of the vehicle in everyday driving use. Closely related to that is the highway mileage of our car. And this latter number is one we can actually measure with some accuracy. That's because it can be measured using the things I mentioned a couple of paragraphs back.

So fill the gas tank to the very top, pick a piece of highway, 70-80 miles, pretty flat (no hills), a day (or night) when traffic is light and there is little head or tailwind. Drive for an hour or so at the speed limit as close as you can making note of the number of starts/stops/ speedups and slowdowns. Fill the gas tank again. Drive back with the same pattern. Fill the gas tank again. Record on paper all the numbers including outside air temperatures taken several times. The resulting mileage figures are good data and a baseline with which to compare any changes you make to the engine whether new or simply restoring the engine to stock or some earlier condition of modifications.

Note that if you make two or more changes between tests you won't know which change helped and how much. That may or may not be important to your objectives. (Not going to get into the subject of designed experiments, an exercise in mathematical statistics, here.)

This is real engineering. Identify the problem correctly and just solve it using tools you can trust. You get real information you can use.

So there are no horsepower or thermal efficiency numbers in the procedure I described. So what? Are you looking for some bragging rights or useful information? Bragging rights? Then go spend a pile of money on electronics, accelerometers, load cells and a bunch of other high tech measuring devices. Neat hobby. I could even get into that. But do you need useful info or a new hobby?

Ed Weldon

To ED thanks

I have done a city and hwy fuel consumption test for the city I got a 58% increase in milage over a normal engine and have a tacho graph as well see pic, I drove around the block 12 times 6 righthand and 6 lefthand, have done some stats and found I was accelerating for approx 20% of the time and idling for only 7% the idle I think is a bit low but I am trying to find the proccedure for such a test the amount of fuel used during this test was approx 50 cc/min over 67.8 km.

The hwy test is approx 34% better on a distantance over 100km.

Bragging rights? in terms of HP its about the same as a stock one now, since someone put silicon in my engine but I have plotted rpm increase per pistion fired under full load and the increase looks fairly linear the engine by seat of my pants seem to pull the car at a constant rate when accelerating.

Can you invent a control/carburretter system where the engine starts with firing one cylinder only and as the rev goes up other cylinders starts firing one by one.This will save gas during low speed running and at signal lights.

You can get some of the way towards that with a modular opposed piston opposed cylinder engine.

Thanks. My proposal is to have 3 or 5 cylinder engine with interacting ignition and carburreter systems.When you start the engine the middle cylinder only fires then when you rev up weak mixture will flow to two cylinders on either side.When the speed further increases rich mixture will flow to the two cylinders.If you want further increase the other two cylinders on either end will come into action by firing first weak and later rich mixture. The odd number of cylinders will balance the forces on the crank shaft by adjusting the air/fuel ratio in each cylinder.

G'day JohnDG, gals, guys & gurus,

I don't know if you have seen them but the opposed piston opposed cylinder engine in your link looks very much like a flattened out Napier Deltic engine as shown in the animation on the right courtesy of Wikipedia. The other similarity is the positioning of the inlet and outlet ports which as far as I can see are identical and therefore nothing new.

The Deltic was developed in the 1940s for the Royal Navy to use in high speed torpedo boats but they also found their way into railway diesel electric locomotives.

Anyway, with the flattened out engine in the video I doubt it would have been more efficient or powerful than a Deltic due to the complex arrangement that links the outer pistons to the crank shaft in the middle. Folding the whole thing into a triangle that utilizes three linked crank shafts makes far more sense to me.

They were fairly fiddly to maintain which is probably why the use of them has petered out over the years and I don't know if they are still in use. Also given the complexity of the engine in the video I would hazard to say that they would be even more difficult and fiddly to maintain than the Deltic engines so I can't see them being used where simpler less complex engine designs could be used.

Regards, masu

An another thing is that I had to write my own program in Microsofts Quick Basic ver 4.5 to control the timing of my engine via the printer port.

My timing curve is a straight line 0 deg at 400 to 35 deg BTDC at 10000 rpm yes a straight line.

Here is I pic of the screen output from the laptop in my car.

Anyway its been T-Boned off the road 3 years ago makes me wonder..

I had stopped fair and square in front of oncoming ute if only I was in 2nd gear not overdrive I could have avoided it by flooring it.