Van Bortel Blog
- Sunday, May 26, 2013
Our turbo-chargers are just small air compressors. Their sole job is to send compressed air to the engine; that's how we maintain (or increase) power at altitude. There are other engine components that control turbo-charger output, but the turbo just pumps air. It's that simple.
One problem that we face in turbo-charged aircraft is the temperature of the compressed air. Anytime air is compressed the temperature is increased. For our piston engines, this produces a number of problems. Increased induction air temperature increases cylinder head temps and can lead to detonation, which can caused catastrophic failure of the engine. Hotter air also results in less efficiency. Hotter air is less dense, meaning that there are fewer air molecules entering the cylinders for combustion, which decreases the volumetric efficiency of the air. This works against the efforts of the turbo-charger.
There are a few ways to deal with the problem of increased air temperature. Some are better than others. The most simplistic option is to decrease the performance of the turbo-charger, i.e. not compress as much air. Certainly this isn’t the best option, we want to cram as much air into the cylinders that we possibly can, after all, it's all about speed & power, right? Another option might be to increase the fuel flow into the cylinders. Now this fuel isn’t being burned, as it is atomized it absorbs heat and carries it out the exhaust valve, thus providing a cooling function. This isn’t a great solution because it increases fuel burn and emissions. Today, the major players in aircraft engines such as TCM and Tornado Alley Turbo use intercoolers to help offset the increased induction air temperature, but it's important to note that the compressed air is still warmer than it was before entering the turbo-charger.
An intercooler is simply a heat exchanger. In a heat exchanger, two fluids do not specifically come in contact, but a transfer of heat (energy) takes place between them. Hot, compressed air exiting the turbo then flows through the intercooler where heat is transferred to the tubes & fins of the intercooler (think of the radiator on your car). Ambient air then flows over & through the fins of the intercooler and absorbs the heat from the intercooler and sends it overboard.
The amount that an intercooler cools compressed air depends on a number of things. First, the design of the intercooler comes into play. What is the size of the intercoolers surface area? How many tubes? Fins? Second, where is the intercooler installed? Can a large amount of air flow over the intercooler with ease? Aircraft speed also plays a part. Fast cruises at low angles of attack provide more cooling airflow then low airspeeds and high angles of attack. As you can imagine, ambient air temperature plays a huge role in determining the effectiveness of an intercooler. Simply put, the greater the difference in the compressed air temperature and the ambient temperature, the easier it is for the intercooler to transfer heat.
Many aircraft owners perform their own regular oil changes. When the cowling is off, take a close look at the intercoolers and the areas surrounding them. Take a look at the surface area of the intercooler. Are there cracks or damage to the fins? What about the baffling area, is it sealed properly? Any of these can lead to a reduction of the intercoolers effectiveness. Consult your technician or aircraft maintenance representative if you notice any potential issues.
In a nutshell, intercoolers are in integral part in today’s modern high performance piston aircraft engines. As a pilot, it’s important to know in detail how the various components in your aircraft work. Next time you’re in the shop, ask your technician for a quick tutorial on the system.
"Thank you for taking the time to help me get my dream airplane" — C.P.