Supercharged
Supercharged
TDC
Kevin Cameron
WHY NOT SUPERCHARGED STREETbikes? This would be an entirely new kind of superbike-one with power equal to current Open-class engines, but with the lighter weight and greater agility of present 600cc sportbikes.
We are accustomed to motorcycle engines making their power either through displacement (low-revving cruisers) or rpm (sportbikes), but always in both cases being limited in torque by the atmosphere’s modest ability to fill cylinders.
Supercharging changes that. You say your 600 makes 120 horsepower at the crank? Presto puffo, blow into its intakes with twice atmospheric pressure, supplied by any of several kinds of pumps, and now you have 240 bhp and an accompanying bulge in the torque curve. This is because engine power is almost
directly proportional to intake pressure, Supercharging is any system that uses a compressor to raise intake pressure above atmospheric. Want even more? Dial up three atmospheres of intake pressure from our blower system and now we have 360 bhp. You can go as far as the mechanical strength of the engine, its cooling system and the threat of detonation will let you.
How far is that? During the turbo years in Formula One auto racing, the 1500cc Megatron/BMW engine ran in one quaiifying session at more than 1300 horsepower. If you do the numbers, you get 867 bhp per liter. For our usual sportbike displacements of 600, 750 and 1 lOOcc, that translates to 520, 650 and 953 bhp. Because we can’t afford full F-l technology, let’s cut those numbers in half. The result is still a mind-boggling 260-horsepower 600cc sportbike!
Turbocharging is just one type of supercharging-its full name is turbo-supercharging, a technology developed for aviation just after World War I. Turbochargers are small centrifugal blowers, powered by turbines driven by the engine’s exhaust gas. Other types of supercharging use mechanically driven compressors or, in the case of the Comprex device, employ pulses of exhaust pressure to compress individual slugs of intake air. Mechanically driven superchargers take some power from the crank, and all blowers heat the intake air.
In the present era of weight consciousness, supercharging offers the prospect of smaller, lighter engines for a given horsepower. At least one maker of boat motors is shortly expected to reveal a compact four-stroke outboard of unusually small design because it gets its power not from displacement, but from supercharging, This will be important in selling fourstroke engines to marine buyers accustomed to compact two-strokes.
BMW, whose winning flat-Twin 500 Grand Prix bikes of the late 1930s were supercharged, may again be playing with this concept. Historical tradition is considered an important part of brand identity—itself a pillar of owner loyalty. It is suggested that a “kompressor” model of the current 1150 flat-Twin could make 150 bhp, thereby becoming instantly performance-competitive with such machines as Ducati’s fabled 996. Yes, it’s possible.
Why not do it? Turbos on cars fell from favor because of turbo lag and the understandable unreliability of anything that turns 150,000 rpm while glowing white-hot. Turbo performance was replaced by a new generation of non-turbo four-valve auto engines with comparatively instant throttle response. At the same time, the collapse of OPEC’s fuel prices put bigger engines back on the market. This we know: Gas prices will rise again, and smaller, more fuel-efficient engines will replace the monsters now in 12-mpg SUVs. Supercharging is one way to pull big power from small, light and efficient engines.
The major problem is detonation. The tighter you squeeze the charge in the combustion chamber—either through a high compression ratio or by stuffing more in with a blower-the more likely destructive engine knock becomes. To
prevent this, you reduce the engine’s static compression ratio. This, in turn, causes a loss of economy in ordinary “off-boost” driving. This is unwanted in Ann Arbor (home of the EPA). The turbo F-l cars of the 1980s ran around 8:1 compression on toluene-based fuel, and the supercharged aircraft engines of WWII ran 6.5-7.0:1 compression with 100/120 or 115/145 fuel. On modern “Knock-O” pump gasolines, conservatism would be the watchword.
How will the marine people get around this? Marine engines spend a lot of time at full throttle, and they don’t yet face consumption and emissions regulations as tough as those on the highways. Therefore, they can set compression for best performance at full throttle.
How about variable compression ratio-something like Saab’s tilting cylinder block engine? That could be a really cool combo: high compression for efificient fuel use in off-boost driving, low compression for knock-free operation at full throttle. Trouble is, extra parts and features mean extra cost,
What else? Another way to avoid detonation at high boost is to inject something that cools the charge-like a 50/50 mixture of water and methanol. While perfectly workable, this adds complexity and forces the owner to remember to fill two tanks with different fuels. We can also directly cool the blower-heated air with a bulky intercooler. With detonation sensors on our engine, we could just let the electronics handle it. If the engine begins to tinkle on boost, the computer will retard the timing enough to stop the knock. If the exhaust valves begin to get too hot from retarded timing, the dash display will flash a message that says “Max boost not available now. Please refuel with 93-octane.”
Other problems? When you miss a shift under boost, the pressure drives your tach into the red much faster, but rev limiters should handle this. In the old days the big question was whether the blower should suck from the carburetor or blow into it, but fuel-injection makes this “problem” disappear. In fact, the level of computer electronic control available today on production vehicles is probably similar to what was used to protect GP car engines in the turbo era. Why should a powerful motorcycle have to weigh as much as two linebackers? Supercharging? I think BMW, for one, should do it. □
