12/15/2006

A FORCE TO RECKON WITH!

Last month I talked about how, compared to the past, current street engines can make so much more power for their size and still stay together. In this issue I’m going to discuss the application of force in engine dynamics.  Keep in mind that we are talking specifically about the most popular automotive engine design, the four-stroke engine.

Four-stroke engines only accept power for very brief periods of time through a single crank rotation. They’re called non-continuous combustion engines for this reason. To break it down in very simple terms, there’s basically two parts to an engine. The upper part that breathes, and the lower part that turns a portion of the combustion energy into usable rotational power. This lets you connect it to the transmission and tires (or propeller for you boat freaks). The breathing part includes the basic stuff - heads, cam(s), intake, and the lower part is obviously the short block.

The upper part of the engine, which regulates its volumetric efficiency (VE), can also include a power adder, like a supercharger, turbocharger or nitrous oxide (which can make the engine act like it has more than 100% VE, because of the high and cold oxygen content). So when you build a performance engine from the top end standpoint, this is where you make the power - by making it breathe more efficiently. One option to get the engine to breathe better is to modify the valve train (and accompanying reciprocating components) so that the engine can rev substantially higher than it was originally designed to do. The valve train components must be significantly stronger in order to withstand the incredible stresses incurred by high rpm acceleration. A big advantage to power adders is that you don’t need to rev the engine nearly as much to obtain the desired power. This means you may be able to save thousands of dollars by not having to buy parts specifically for high rpm operation, and yet still produce well over two horsepower per cubic inch with a blower type of power adder. But depending on your desired power level, you shouldn’t simply use stock valve train parts either.

Although I’ll be talking about many other aspects of adding boost to an engine in future issues, one of the most basic things to understand about a supercharged or turbocharged engine is how the power is applied to the rotating assembly -- the crankshaft, piston and rod. Remember, with a four-stroke engine we only get to apply power to the engine every other stroke, and for a very short period of its rotation.  This power duration is so limited that even if it can be increased by a few degrees rotation, substantially more power can be extracted from the engine while adding virtually no more stress – which I believe we all can agree would be a useful goal. 

These days a normally aspirated engine can easily make well over one horsepower per cubic inch. It needs high compression to do this, and the more the better. The high compression ratio does indeed increase volumetric efficiency and power output, but more of the force is applied to the rotating assembly in the early part of the power stroke (first 90°).  Contrast this with a supercharged engine, which continues to convert power way past 90° of rotation.

In reciprocating engines, how the force is applied is almost more important than how much is applied. Think about it this way. If a 300-pound power lifter forcefully stands on a bicycle pedal at top dead center (all the way up) or bottom dead center (all the way down), it doesn’t really matter how much force he applies, because the bike isn’t going to go anywhere. But if a small child applies power to the same bicycle at 90° (halfway down) he may build up some pretty good speed. So, where and how long the power is applied to the piston makes all the difference in the world in terms of net power output.

The reasons why the power is applied much longer in a blown application are the low compression ratio and the massive quantities of air provided by the power adder. Since the power adder can make the engine act as though it is tens of thousands of feet below sea level, it can flow substantial air for its displacement (CID) and produce massive power. Also, because of the high manifold pressure (boost), the intake stroke starts substantially sooner and ends later. It’s also true that with the lower compression ratio, the fuel conversion efficiency is slightly lower. What this means is that for every ounce of fuel, a bit less energy is extracted. But a big benefit with the lower compression ratio is that the peak loads on the rotating assembly are substantially reduced. 

This is extremely important to engine longevity, as the rotating centrifugal force of the crankshaft and rod assemblies are really where most of the load in the short block is contained. As an example, as an engine is revved from 5,000 to just less than 7,000 rpm, dependent on rod ratios and without a cylinder head attached or any combustion energy associated, the stresses more than double on the rotating assembly just from acceleration forces. And what’s most important is that a big-block engine that makes 700 hp at only 6,000 rpm may easily only experience less than 20% of its peak load derived from the combustion of the fuel, versus the acceleration and stopping of the piston assembly itself.

Obviously, in the above example if the horsepower is doubled but the engine RPM remains constant, the total load on the rotating assembly is only affected by a very small amount. Although this explanation is at a very high level and not very specific, it can easily explain why so much power can be made with not a lot of engine, when using a good power adder.

In the following issues I’ll cover a lot of other questions people have asked over the years, concerning today’s big power levels, and how they work. 

 

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