The Basics Of Power

[Hugh Janus]

Creating horsepower takes cylinder displacement, stroke-averaged net combustion pressure, and rpm. (Edoardo Nicolino/123rf.com /)

Cylinder displacement, stroke-averaged net combustion pressure, and rpm. These are the basic variables that create horsepower. Cylinder displacement is obvious, for it tells us something about the volume of fuel-air mixture an engine can pump into itself. In an un-supercharged engine each cubic inch of mixture (at atmospheric pressure) contains a definite amount of chemical energy. The bigger the cylinder, the greater the push we can expect it to give its piston.

Although we intuitively expect to gain power in proportion to displacement when installing a big-bore kit on a classic engine such as a Kawasaki Z1, the result is less than we expect because the engine’s valves were sized for its original displacement. Our big-bore kit “sucks” harder but the valves are really a bit small for the larger 1,000 or 1,100cc engine we’ve built, so they restrict the flow somewhat. The result is a less-than-proportional gain in power. 

As an example of a more balanced approach to a displacement increase, KTM increased the diameter of its 790′s intake and exhaust valves by 1mm each when it recently enlarged that engine’s displacement to 890cc.

Stroke-averaged net combustion pressure is an abstraction—in reality, combustion pressure is constantly changing, beginning to rise shortly after the spark ignites the mixture, reaching a maximum at about 11 degrees after top dead center (ATDC), and then falling as the piston descends on its power stroke. That constantly changing combustion gas pressure can be measured by fast, accurate high-pressure microphones, but we make a useful abstraction of it to simplify our thinking.

Therefore stroke-averaged net combustion pressure is abbreviated to BMEP (brake mean effective pressure) which averages the complex, changing actual combustion pressure to a constant pressure which, if it acted over the entire piston stroke, would produce the same power at the crankshaft.

Notice especially the word “net.” It is there to tell us that this BMEP is what is left after normal engine losses—the viscous friction of bearings and sliding pistons, pumping loss, etc.—have been subtracted from the raw energy of combustion gas. That is normal in dynamometer testing—the power we get at the sprocket or shaft is what is left after losses are subtracted.

BMEP is the sum of many parts.

1. How well does the engine fill its cylinders? The more completely the cylinders are filled, the greater the BMEP. Cylinder filling is measured as “volumetric efficiency”—which compares cylinder displacement to the volume of fuel-air mixture actually trapped in the cylinder after the intake valves close. One hundred-percent volumetric efficiency means that the cylinder is filled completely, but wave effects in the intake process can sometimes increase this to as much as 125 percent.
2. How efficiently does it burn that charge? There is some flame quenching on metal surfaces, and some injected fuel droplets may be too big to burn completely, or may hit the cylinder wall and be lost into the crankcase—swept away by the piston rings. 
3. How much energy is consumed by friction, heat loss, and the forced rapid movement of air in the crankcase? (One Japanese maker gained 5 hp in an 1,100cc four by “streamlining” this movement.) Engineers seek to make combustion rapid, as the longer super-hot flaming combustion gas is held between piston and head, the more heat flows out of that gas and into the metal parts, from which it must then be removed by the engine’s cooling system. At low or cruising throttle more energy is consumed by pumping loss, which is associated with making the engine perform the work of pulling against low manifold pressure. Mainly because of this, engines deliver better gas mileage on the interstate at 75 mph than they do tootling along country roads at 40 mph.

In general, engine BMEP has increased steadily as manufacturers have sought to produce more power from the various classes of engines they produce. This effect is obvious in the sportbike world, but also operates in the world of touring, where ever-heavier bikes need more oomph to keep up with speeding multilane traffic.

The third major element in engine power is rpm—revolutions per minute, which is a measure of how often our engine can perform its energy-release cycle. There are limits to the benefit of using rpm to boost power, because as revs rise so does friction loss. The sports management organization Dorna, which administers MotoGP, did not want its manufacturers going mad as F1 had done, pushing the peak revs of big V-10 engines to 20,000. Being interested in such technology, I had been delighted to see Honda push its five-cylinder 125 GP bike engine to 21,000 back in the 1960s, but pouring money into technology that has no marketplace application is a poor investment. For that reason Dorna set a limit to how far MotoGP could go in raising rpm through use of bigger pistons and ever-shorter strokes. In F1 piston diameter grew to be 2.3 times the stroke length, slowing combustion and increasing heat loss. Dorna therefore set a limit at 81x48.5mm, or a bore/stroke ratio of 1.67.

The need to meet more stringent emissions levels has required some rethinking about BMEP. The hotter combustion is made (by higher volumetric efficiency, higher compression, and more accurate control of mixture), the more oxides of nitrogen are produced—combustion hot enough to actually burn atmospheric nitrogen. Dyno operators are familiar with the sharp tang of such oxides. Another ill effect of higher combustion temperature is losses from molecular dissociation and from energy in very hot combustion gas being less available for piston-pushing duty because it partly takes the form of molecular rotation and vibration. This is a major part of how operation on lean mixture reduces fuel consumption. Leaning the mixture reduces peak combustion temperature, lessening the losses from the just-described effects. What pushes pistons is the velocity of hot gas molecules, physically hitting the piston and giving it a shove. But energy in the form of molecular rotations and vibrations, which appear increasingly at higher temperature, doesn’t push pistons. That energy goes out the exhaust pipe.

In supercharging engines we boost power by compressing the mixture, in effect forcing, say, 300cc of mixture into a 250cc cylinder. But because of the above effects we don’t get the full 20-percent power boost that dividing 250 into 300 says we should. The hotter we make combustion by conventional tuning techniques, the greater the loss from these effects. 

Supercharging in effect “stuffs” a larger volume of mixture into a cylinder than its actual displacement. (Jeff Allen /)

Therefore manufacturers get busy with test engines on instrumented dynos, with hundreds of possible fueling and ignition maps, to find ways to get the power their intended market demands, in a form that meets US EPA or European Commission standards.

I believe we will be seeing some slow reduction in motorcycle engine BMEP as it becomes necessary to reduce peak combustion temperature by use of leaner mixtures. To deliver or exceed the power of last year’s model, while meeting relevant emissions limits, it may prove necessary to compensate for loss of some BMEP by increasing the other two major variables in engine power: displacement and rpm.

As a former chief engineer at Harley, Earl Werner, once said, “The job of engineering is not to seek abstract extremes but rather to deliver what people actually want.”

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