Tuesday, November 28, 2006

AV - Honest Engines

2180cc equals the total swept volume, or about 133 cubic inches. (Conversion factor: .06102 x cc = cu. inches)

133cid represents all four cylinders. Since this is an Otto Cycle engine and there are only four cylinders, there are only two intake cycles per revolution so we're really only interested in half the swept volume or 66.5 cu. inches.

Airflow is normally measured in cubic feet. There are 1728 cubic inches in a cubic foot and since one revolution pumps 66.5 cubic inches of air that's equal to .03849 cu. feet.

Optimum prop speed is across an rpm range from about 2250 to 2850.

2250 x .03849 = 86.5 cu. feet 2850 x .03849 = 109.7

That's for 100% volumetric efficiency, of course. And that can't happen unless the engine is supercharged. But at those speeds, with the valve train properly set up, a VE of 80% is possible if we can keep the temperatures down. That would give us a flow-rate of about 87 cubic feet per minute.

Air weighs approximately 0.08071 pounds per cubic foot so every minute we are pumping 7.081 pounds of air. (Keep in mind, this is for AIR, not OXYGEN)

The stoichiometric ratio for gasoline and air is 14.7:1 so every minute I'm burning about .481 pounds of gasoline, which means I will burn about 28.9 pounds of gasoline per hour at a throttle setting that produces 2850 rpm at a manifold pressure of about 3" of mercury. Since gasoline weighs about 6 pounds per gallon I will be burning about 4.8 gallons per hour. (Hint: Stoke is based on the mass of the fuel & oxidizer rather than its volume.)

Based on accepted standards for Specific Fuel Consumption (i.e., .5 lbs per horsepower-hour for a well designed air cooled engine) my 2180cc engine should be producing about 57.8 hp. And it probably will. But only during take off. That's because VW has not increased the size of the fins on its cylinder heads since it introduced the heads we all now use. Originally, the heads were designed for the 40 horsepower 1300cc engine. VW eventually bored the engine out to `1600' (actual swept volume 1584cc) but kept the same heads. The fin-area of the heads puts a thermal limitation on the output of any VW engine NO MATTER THE SIZE. The thermal limit is determined by the cylinder head temperatures, which should be kept at or below 325F. if you want the valves to last.

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There are lots of variables in any on-paper exercise of this sort but even when everything is selected for the optimum output the difference is barely 10%. In reality, the nominal output of an on- paper engine is usually quite a bit better than anything you'll get from the real thing. Paper engines are goals to shoot for. So you do the best you can, isolating one factor at a time, making modifications to improve that single factor and running another series of tests. It teaches you a lot about engines. And about yourself.

On a broader scale, one of the most interesting aspects of the figures above is that they are in general agreement with figures produced by actual torque measurements on the Whatley test stand recently (Fall, 2003) discussed on various flying Volkswagen groups.

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The numbers are interesting but the engines themselves are more so. And more fun. I've spelled out the figures I used so you plug in your own numbers for displacement and rpm, which I think you'll find to be the most significant factors. But there's a good chance you'll be mislead and the reason deserves mention.

As you increase the rpm the on-paper horsepower will rise dramatically. All else being equal (it's not, but bear with me here), you'll tend to fix your prop-speed at something more than 2850 rpm. But actual tests with real engines and real propellers shows that the efficiency of your prop falls at a faster rate than can be offset by any increase in horsepower. This is definitely one of the tricky bits because you end up with a `strong 70hp' engine that is producing barely 35hp-worth of thrust. I believe you will find that you can fly farther, faster, by using the longest prop your engine can swing.

Rather than give you a Break Mean Effective Pressure I used a nominal manifold pressure that is conservative compared to real aircraft engines. I used a conservative figure because I'd like to save you the trouble of blowing up an engine :-) Manifold pressure is also easy to measure whereas BMEP is not.

Increasing your compression ratio will improve your BMEP and thus the output of the engine but on the VW, as the output increases the heating effects become critical and announce themselves by a catastrophic drop in your volumetric efficiency... just before you eat it. That's because the incoming fuel-air charge absorbs a lot of that heat, which gives you a very lean burn. At high compression ratios a lean mix leads to detonation, quick like a bunny, and there you are surrounded by white smoke with a blown engine hanging off the test stand.

Which is better than having it happen when you're half way to Catalina. So be cool.


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