The Comprex: The Other Compressor

Appendix J

Sometime back when I was perusing the Bosch Automotive bible looking up compressor maps for positive displacement superchargers, I came upon the rather odd image below. It's called a Comprex, something like a hybrid mutant of a turbo and a supercharger, but better than both-at least in theory.

While not a new invention, we don't really see much of the Comprex, since it's mainly used in large marine and earth-moving diesel engines and, in some cases, smaller passenger car applications. But my interest was piqued by the fact that the Ferrari Formula One team played with a Comprex on their early-80s turbo cars, with better results than conventional turbocharging.

Officially called the Pressure Wave Supercharger, a Comprex is basically a stationary drum casing with a lost-wax cast straight-vane rotor spinning inside the drum, creating boost. Think of it as a wide water wheel inside a drum.

Like a turbo, the casing and rotor aren't in contact, but the clearance between the two is kept to a minimum (barring thermal expansion and creep) to prevent boost leakage. The synchronized belt-driven rotor is powered by the crank, moving around four to five times faster than the engine, but only drawing enough to overcome the frictional losses of the assembly. This means the Comprex doesn't suck power away from the engine to do the work of compression. The compression is done by the exhaust gases like a turbo, which is essentially free energy.

So if you're not using the engine to compress air like a supercharger and not driving the rotating assembly with exhaust gases like a turbo, what is doing the compressing? This is where the 'pressure wave' portion of the name comes in. Incoming ambient air is compressed by using the pressure wave from the exhaust gas.

Each end of the drum has two different-sized ports, connected by ducts for air or exhaust gas to enter and exit. On one side of the drum, air enters from the intake at near-ambient pressure and exits at boost pressure to the intake manifold, while on the other side, exhaust gas from the exhaust manifold enters at high pressure and exits to the tailpipe at lower pressures. How compression is done is the hard part to explain.

The process starts as a given channel on the rotor already filled with ambient intake air (I'll tell you how it's filled later). Neither end of this channel is lined up with a port, so it's completely sealed off by each end of the drum. As the drum rotates, the port on the right side, a smaller high-pressure exhaust orifice, is exposed first to let in the just-combusted gases, which introduces a compression or shock wave into the channel. The shock wave propagates at the localized speed of sound and pushes fresh air against the left wall of the drum, which is still closed and thus compressing the charge. These compression waves are not on account of the individual pulses of each cylinder firing, just the rapid introduction of two gases at different pressures.

As the charge compresses, it makes space, allowing the exhaust gas to enter the channel. Since the shock wave is traveling so fast, the two gases never mix. By this point, the channel has rotated to the high-pressure air port leading to the intake manifold. Although rated for the same mass flow rate, the smaller port is sized so that the compressed air exits at a much lower velocity. This deceleration of the compressed air causes a secondary shock wave to propagate toward the right (or exhaust) side, which compresses the fresh air further. This way, the boosted air going into the engine is actually at a higher pressure than the exhaust gases.

As this secondary compression wave reaches the right side of the drum, the high-pressure gas port closes, causing the compression wave to reflect back as an expansion wave, pushing most of the compressed air out and closing that port. By now, the low-pressure exhaust port on the right is exposed, letting the now slightly pressurized exhaust out into the tailpipe. This causes another series of expansion and compression waves that ultimately help pull in and completely fill the channel with fresh air, which brings us back to step one.

In its basic form, the Comprex only works well under specific conditions, because the speed of the compression waves depend on EGT (exhaust gas temperature), which fluctuates with torque and not engine speed. To stretch out the operating range to that of most engines, specially designed pockets are added to the intake and exhaust sides of the drum to compensate for the varying range of EGT and thus the wave propagation speed. Boost pressure is controlled by a conventional wastegate on the high-pressure exhaust duct.

The main advantage of the Comprex is the combined effect of response and efficiency. Since these pressure waves travel at the speed of sound, any change in load will cause an instant reaction in boost. Unlike a turbo, which has to wait for the compressor wheel to spin up to speed. It can also achieve apparent compressor efficiencies of up to 75 percent, compared to 60 percent seen in the best positive displacement compressors. The Comprex also turns much slower than a conventional turbo, meaning reduced loads on the rotor.

So if it's so good, why is everyone still using turbos? For one, packaging. The Comprex isn't something you can just bolt onto a manifold and throw piping at. Also, the rotor, drum and port dimensions all need to be tailored specifically for a given engine.

Another concern is materials. The Comprex is widely used on diesel applications because of the lower EGTs. Making the Comprex tolerate higher EGTs from a gasoline engine is only now becoming affordable and practical. Flow reversal for the exhaust may also be an issue, although newer cross-flow and radial Comprex superchargers exist for industrial applications.

I'm just waiting for some demented soul to strap one of these compressors on, so we can see just how good it really is.

Jay Chen
Engineering Editor

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