Piston Engines - Performance and Power Augmentation — Page 153, Lesson 200

Right, let's pick this up with the turbo-lag definition, because that's the perfect bridge from what we've been discussing about the wastegate and the APC. Turbo-lag is the result of the time it takes to speed up the turbine and compressor after the signal of low compressor output has been sent to the Absolute Pressure Controller, the APC, and the wastegate actuator has reacted by closing the wastegate. Let me unpack that for you. When you demand more power, the compressor initially isn't delivering enough pressure. That low output is sensed by the APC. The APC then sends a signal, and the wastegate actuator reacts by closing the wastegate. Closing the wastegate forces all the exhaust gas through the turbine, which is what spools it up. But that whole sequence—sensing, signalling, actuating, and physically spinning the turbine and compressor up to speed—takes time. That delay, between you opening the throttle and the boost actually arriving, is turbo-lag. It's not a fault; it's the inherent response time of the system. Now, I want to consolidate everything we've covered on supercharging versus turbocharging, because the book gives us a clean summary table, and I want you to see the contrasts side by side. First, the drive. A supercharger is internally driven, meaning it's geared off the engine itself. A turbocharger is externally driven, powered by the exhaust gases flowing through the turbine. Second, the speed control. The supercharger's rotational speed is controlled by engine rpm—the faster the engine turns, the faster the supercharger turns. The turbocharger's rotational speed is controlled by the wastegate position, which we've just talked about. Third, what they compress. The supercharger compresses the mixture—that's the fuel and air together, before it enters the cylinders. The turbocharger compresses air only, and the fuel is added downstream. Fourth, the control system. The supercharger uses an ABC, the Automatic Boost Control, which senses manifold pressure and controls the throttle. The turbocharger uses the APC, the Absolute Pressure Controller, which senses compressor discharge pressure and controls the wastegate. So the supercharger regulates by throttling the engine; the turbocharger regulates by bypassing exhaust gas. Fifth, the pressure relationship. With a supercharger, the compressor discharge pressure is the same as the manifold pressure. With a turbocharger, the compressor discharge pressure is greater than the manifold pressure. That's a key distinction—the turbocharger builds pressure ahead of the throttle, and the throttle then meters it down to manifold pressure. Sixth, the throttle. In both cases, the throttle controls manifold pressure. That's the common thread. And seventh, the exhaust back pressure in the climb. The supercharger gives you decreased exhaust back pressure in the climb, because the engine is driving the compressor directly. The turbocharger gives you increased exhaust back pressure in the climb, because the exhaust is being used to drive the turbine, and that creates resistance in the exhaust system. Now, let's move to diesel engines, because the same principles apply there. Diesel engines also suffer from a loss of volumetric efficiency with altitude, on high-elevation take-offs, and on hotter-than-standard days. Volumetric efficiency, remember, is how well the engine fills its cylinders with air. At altitude the air is thinner, at high elevation the same applies, and on hot days the air is less dense—so in all three cases, the engine can't get enough air into the cylinders, and power falls off. For this reason, turbochargers—which the book calls external superchargers—may be fitted to diesel engines to improve performance, in exactly the same way as on a conventional piston engine. The turbocharger forces more air in, restoring that volumetric efficiency. And finally, intercoolers are also employed to restore density after compression. Here's the logic: when you compress air, it heats up, and hot air is less dense. So the turbocharger gives you high-pressure air, but it's hot. The intercooler cools that compressed air back down, which restores its density. Denser air means more oxygen in each cylinder, which means more fuel can be burned and more power produced. So the intercooler isn't an optional extra—it's what makes the turbocharger's benefit fully usable. That's the complete picture: turbo-lag as the response delay, the seven-point contrast between supercharger and turbocharger, and how the same turbocharging and intercooling logic applies to diesels.