Not dissing anyone here, in fact the contrary. In this short post we’re going to look at the implementation of direct injection technology on two ends of the performance spectrum.

I’ve been on the record saying that I am a huge fan of direct injection technology (though it has existed in diesel applications for ages) as it transitions to the gasoline and performance arena because it raises the potential volumetric efficiency of the same sized engine by as much as 15% according to Audi‘s documentation.

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Direct injection technology injects fuel (diesel, gasoline, E85…etc) directly into the combustion chamber after the intake valve closes during the compression stroke (rather than during the induction stroke). This allows for a few things to happen:

  • Your intake manifold is 100% dry at all times and things like manifold backfires are less likely.
  • Less hassle and cleaning to do with fuel and deposit accumulation inside the intake manifold, EGR, the backside of the intake valve, the throttle body …etc
  • In traditional injection setups, anything you inject into the intake air stream (such as fuel, water injection, methanol cooling, or even nitrous oxide, or exhaust gas through your EGR) displaces part of your intake volume and dilutes your intake mixture down from being 100% fresh air. Removing the fuel injection out of the intake stream allows for a few percents more fresh air being introduced into the same volume engine boosting its volumetric efficiency.
  • Since the fuel is delivered directly into the combustion chamber, more fuel cooling is applied to the intake air charge, rather than being wasted cooling the walls of the intake manifold.
  • In order for fuel to be able to be sprayed into the chamber in the first place, it needs to be injected at very high pressures… the side effect of this ‘need’ is that the fuel gets injected with a much better atomization due to the higher pressure which improves air and fuel mixing inside the cylinder.
  • The combination of not wasting fuel inside the intake manifold, and not needing to ‘over richen’ the mixture now that we have better fuel atomization due to a higher line pressure means that less fuel is needed overall for the same power level.
  • Last, but not least, and maybe even the MOST important reason of all… on high boost applications, if you have a 100% dry manifold, then no matter how high your boost pressure or how heat soaked your intercooler, since there is no fuel present in the intake manifold, since the fuel doesn’t get injected until the compression stroke starts, since we have 100% control over our fuel timing inside the chamber … all of this combined means that you will never run into pre-ignition because there is nothing there to ignite!
This last point is in my opinion the most important aspect of Direct Injection setups for high boost applications, they not only produce more power per psi and per cubic inch of displacement of a traditional port injection setup; but also, they are SAFER at very high boost and power levels than the old technology setups.
Below is a video produced by an injector cleaner company that shows you just how dirty a ‘wet’ intake manifold can get due to fuel, and oil deposits dirtying up the manifold. As you can see here, a lot of buildup can potentially also reduce air flow over the backside of the valve (which is the primary restriction in the intake system being the smallest radius and forcing the air to make a sharp turn around the backside of the valve), as well as potentially affecting proper valve seal.
Here are two examples of direct injection applications on two ends of the performance spectrum:
This is a direct injected supercharged diesel application delivering 98hp from just a 1.2 litre engine (which is pretty high considering the energy density of diesel is lower than that of gasoline and the lack of spark ignition reduces the overall efficiency of combustion).
This is the economy end of the spectrum where direct injection is used to increase volumetric efficiency, superchargers are used to downsize engine size and the combination of the two technologies can deliver a high horsepower, but low weight, small size and low emissions package for a mass market econo-car.
2- The new Audi A6 / A7 3.0 litre TFSI power plant
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The new 3.0 litre TFSI power plant from Audio tosses out the typical Audi turbocharger setup and replaces it with a centrally mounted supercharger between the two banks of the V6 engine. The supercharger is geared for 11.6 psi and fed through a factory designed cold air intake fed from the front grill and the intake charge then is twin-intercooled before being fed back to the intake manifold.
Notice I didn’t say anything about a genorously sized throttle body? … smart engineers don’t only use a new technological advancement to replace old parts, but rather they ask themselves this:
“Now that I have this new Direct Injection Gasoline technology, how can I rethink the entire system to make it better?”
Since the injection timing is now precisely controlled through direct injection, audi’s gasoline engine now throws away the throttle body all together and regulates power delivery through regulating fuel and ignition timing to optimize engine power delivery exactly like a diesel engine does!!!!
Here’s Audi’s official definition of their TFSI system:

FSI® Direct Injection technology increases the torque and power of spark-ignition engines, making them as much as 15 percent more economical and reducing exhaust emissions. In contrast to conventional intake manifold injection on conventional spark-ignition engines, FSI engines inject fuel directly into the combustion chambers. This technology dispenses with the throttle plate, “unthrottling” the engine, reducing heat loss, and thus increasing output while reducing fuel consumption.

The system uses two charge-air supply modes: stratified charge at partial load and homogeneous operation at full load. In the stratified charge mode, a combustible fuel-air mixture is only produced in a defined zone around the spark plug. The engine management electronics monitor engine load and adjust injection timing, pressure, quantity parameters as well as the air flow inside the cylinder via the air intake channel. At full load, FSI increases compression as well as engine efficiency and performance.

The 3.0 litre V6 produces 310hp and 325ft-lbs of torque and delivers a 0-60 time both in the A6 and in the A7 of around 5.2 to 5.3 seconds. Yes 310 horsepower is not all that much to drive around with, but considering the power is delivered through a close ratio 8 speed, twin clut

ch, sequencial gearbox, you can understand the type of mechanical advantage the engine possesses through this unique gearbox thus reducing the need to create astronomically high horsepower figures to deliver such blistering fast acceleration.

 

Direct Injection Supercharged Setups are a trend that I expect to grow in the OEM and racing scenes and there’s nothing to diss about that.