About Turbocharging

When you want to make more power with a motor engine, you need increase the amount of air and fuel it can ingest. One way to do this is to add cylinders (big displacement, think Viper). One way is to increase the number of revolutions the motor completes per minute (high revs, think S2000). One way is to put more air in the cylinder than there is cylinder volume. Turbochargers work on this principal.

Turbochargers work by placing an impeller turbine in the exhaust flow. As the spent exhaust gasses turn the impeller, it turns a compressor wheel (they’re connected by a shaft) to compress the air going into the engine intake. The more air you can force into the engine, the more fuel you can add and the more power you can produce. In the case of the RDX, this means that a 2.3 liter 4 cylinder makes as much power as a 3.5 liter V-6.

This all sounds good in theory, but in practice there are a number of technical hurdles to overcome. The most common issue is throttle response and this is commonly known as Turbo Lag.

Turbo Lag is caused primarily by two things: the turbine impeller needs to have a certain amount of exhaust gas passing over it before it can start producing boost. This is exaggerated if you have a really large turbo (heavier, so it starts spinning slower)) that’s needed for big horsepower gains.

Here’s another issue that needs to be addressed to reduce lag. As you compress the air, it gets hotter, reducing efficiency. As the air heats up, the O2 molecules start moving further apart. This is counter to the idea that we’re placing more air into the cylinder. Now we have the turbo pushing the air at high pressure into the cylinder, but there aren’t that many molecules to support combustion.

This is the reason most manufacturers use an intercooler between the turbo and the engine intake. This large radiator like structure cools the incoming air, thus making intake charge denser (packs more O2 molecules into the cylinder). Most manufacturers place this intercooler in the front of the car by the radiator. This allows it to get a nice flow of cool air to reduce the intake temperatures. However, this results in long plumbing. Why is this problem? All this piping volume needs to be filled with air before it gets to the engine intake. This results in the dreaded turbo lag and throttle response problems that have plagued Turbo cars in the past and one of the reasons Honda has avoided this for so long.

Most of the details on this system are still under wraps, but from the pictures we can see the Acura has placed the intercooler right over the engine to allow for shorter runs between the Turbo and the intake system. The press release states that the RDX is using a Variable Flow Turbo which sounds (to me) like a variable inlet to the turbo compressor impeller. Of course, we could be getting a Variable Vane turbo from Borg-Warner. This design allows the turbine impeller vanes to change their angle of attack depending on RPM. Either will allow the system to increase velocity of the exhaust gasses to help the turbine spool up faster, further reducing lag. Hey, there are just "semi-educated" guesses, but stay tuned for lots more information as it becomes available.

Update 9/29/2006: It looks like my original guess was correct. The Variable Flow Turbo is indeed a dual inlet design and not a variable vane type. See here for more details