Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 11

Horsepower is only produced by the combustion of fuel. Therefore, the more fuel that is burned, the greater the horsepower produced. But the fuel can only be productively burned if it is burned in the engine’s cylinders and there has to be enough air present to do this. We live in an atmosphere that, at sea level, contains 14.7 pounds per square inch (psi) of pressure. This means that an engine’s ability to breath is limited by the vacuum, or suction, created by the intake stroke of the engine’s piston, fed by a maximum pressure of approximately 15 psi. So, even in the most miraculously efficient engine design, there can only be a maximum pressure differential between atmosphere and the intake valve of about 15 psi absolute pressure. And since few of us live and drive on the beach, we have even less than 15 psi to work with. As you rise in altitude atmospheric pressure drops.

Now comes the turbocharger. In the most basic definition, a turbocharger is a device that drives more air into an engine. The 15 psi of absolute pressure is now boosted to something greater, and thus more air is forced into the engine’s cylinders. Now the fuel delivery rate can be increased and more horsepower will be developed. In many cases, a turbocharger can deliver enough additional airflow such that the same size engine can easily produce 100 percent or more horsepower than in its naturally aspirated, or non-boosted, state.

Turbocharger, supercharger, turbo-supercharger, and blower, among other names, are technically correct when referring to any of several different types of devices that perform the principal function in a forced air induction intake system on an internal combustion engine. But among enthusiasts, power wizards, bench racers, and even a few well-read squirrels, supercharger and blower are used synonymously to refer to devices that increase the intake pressure and are powered directly off of the engine’s crankshaft. This book principally concerns itself with the turbocharger, which derives its power from exhaust gas energy through the use of a gas turbine. This difference in technology is the basis for great debate among the performance community about the superiority of superchargers versus turbos. Historically, each device has areas of supremacy over the other, but those areas of application superiority are beginning to narrow, as this book will outline in forthcoming chapters.

This illustration shows a simple aftercooled single turbo system. Let’s follow the intake airflow to see how it works. (Courtesy Honeywell Turbo Technologies)

A. After the intake air passes through the air filter it is routed to the turbocharger’s compressor inlet (1).

B. The air is then compressed, which increases its density—or amount of mass per unit of volume—by the turbo’s compressor section, and is discharged through a boost tube (2).

C. Most of today’s turbo systems use an aftercooler, also known as an intercooler or charge-air cooler, to receive the hot, boosted air as it leaves the compressor. The cooler removes the excess heat, which allows for further air density improvements while also cooling the intake air charge (3).

D. Another boost tube then routes the air from the cooler’s discharge side to the engine’s intake manifold where the air is routed to the intake valves and it enters each cylinder (4).

E. An increased fuel injection charge, based upon the now more dense air charge, is introduced and combustion occurs. After combustion, the exhaust passes through the exhaust valve and into the exhaust manifold (5).

F. The exhaust manifold routes the exhaust gas energy in the form of heat and pressure into the turbine stage of the turbocharger (6).

G. The turbine housing causes backpressure. This backpressure and heat expand against the turbine wheel blades causing the turbine wheel and shaft to spin, which in turn drives the compressor wheel on the intake side of the system. The remaining exhaust then leaves the turbine stage and enters the exhaust system (7).

Turbochargers reign supreme for one simple reason: The turbocharger is the most efficient of all the economically applied forced air induction devices. Unlike the supercharger, which takes power directly off of the crankshaft to drive its compressor, the turbocharger utilizes energy present in the exhaust gas that would otherwise be wasted by simply pumping it into the atmosphere. But even though a turbo uses exhaust gas energy that is commonly referred to as waste energy, there is a cost of using this energy that comes from the increased backpressure formed in the exhaust. Individuals applying turbos to engines frequently overlook the concept of “pumping losses.” Well, there is no free lunch. The relationship between backpressure and turbos are an important concept that must be balanced and is dealt with in greater depth when we get into matching a turbo to a specific engine (Chapter 3).