Friday, June 20, 2014
Engine Mods: It All Starts With Airflow- Part 2
In part one of we went into how an engine moves air, common ways to increase airflow, and why increasing airflow increases operating efficiency. At the end of the last article I mentioned compressors used to increase air charge into an engine and thus increase power and efficiency. Lets go into specifics of the two common types of compressors used and whats needed to make them function, the differences between them, and the benefits of either and why.
First off, there are many ways to compress air, some more complex than others, some better at using input energy to compress air than others, some larger or smaller. Two common types of compressor are used typically on engines. Super-chargers and turbo-chargers are the most often chosen add-on compressor systems. The significant difference between super and turbo-chargers is in how they are driven.
A super-charger, or blower, is a parasitic load run off the the crankshaft that is commonly run via wide cogged belt. A blower takes useful power from the engine to operate it. It works and increases power and efficiency of the engine by compressing and moving more air for less energy than the engine alone can. A blower's output pressure is set by physical component size and the gearing of the drive belt and pulley sets. A blower makes pressure immediately as soon as it is moving. The pressure rise occurs even before the engine starts, and stays at the same pressure through the engine's powerband. This even pressured setup is predictable and easy to build an engine for.
A turbo-charger, on the other hand, has many different characteristics. First and foremost, where the blower is belt driven and takes energy from the crankshaft to run it, a turbo-charger uses waste heat and pressure from the exhaust side of the engine to drive it. As exhaust is directed through the turbo-charger the temperature and pressure drops significantly and mechanical energy is extracted from that waste heat to spin a turbine on a shaft inside the housing at many thousands of RPM. Attached to the other end of this shaft is a similar fan that grabs air from the air filter and crams it into the engines air intake.
Two major downsides with turbo units are the facts that 1) a turbo runs VERY hot and needs warm up and cool-down time to prevent damage, and 2) it takes time for a turbo to spin up and make boost- or pressure- in the intake. As far as the thermal requirements of a turbo, I covered those special needs in another article found (here).
Unlike a blower which make immediate pressure and holds a steady pressure ratio through the RPM range, a turbo needs to 'spool-up' or reach very high speeds before the pressure builds significantly. The time spent waiting for pressure build-up is the 'lag' time which you may hear spoken of in high-performance circles.
A turbocharger will continue to increase its pressure output as engine speed increases instead of staying constant like a blower does. This continual increase in pressure requires additional safety components such as blow-off valves and waste-gates to prevent over-pressuring and thus turbo or engine destruction at higher speeds.
Running a high quality synthetic oil is usually a given with any high-performance engine setup, however with a turbo unit on a machine, the necessity of high temperature and high stress tolerant oils increases dramatically. 'Coking' (burning and turning into coal) of the oil in the bearings of a turbo after shutdown is the number one cause of long term failure of turbochargers. These special care requirements of a turbo is usually not recognized, and thus turbos fail more often than should occur, or would occur if a top quality oil were used and warm-up and cool-down procedures were followed.
A supercharger requires no special additional care, but some types of blower use a special oil just for the compressor. Changing this oil can be a challenge by itself. Like all gear oil systems, if the lubricating fluid is not changed often enough, the metal that has been ground off the gears and bearings soon is acting as abrasive dust carried by the oil to further grind up the moving parts that it came from.
Compression of air causes heat. In a diesel engine, this heat from compression is enough to ignite the fuel that drives the engine. Fighting overheat is tough enough in most engines, adding additional heat from compression before the engines' compression stroke can cause burn-up and burnout of engine components. There are several ways to mitigate the excess heat caused by the compressor, and the next tech article will focus on the most popular methods of lowering intake charge air temerature. Come back and check out the rest of the series of upgrades, how and why Soon then, MachineWhisperer outta here...