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TDI and TSI: are they one and the same?
I’ve just spent an evening driving 250 km through the Quebec forest outside of Montreal in a sports car. I was accompanied by an engineer friend, and the conversation soon turned to the 2.0T engine and its DSG transmission, a fine combination of state-of-the-art technology. We hashed out the for and against regarding direct fuel injection, and tried to decide on the fuel-efficiency of the DSG transmission relative to a manual. We also talked headlamps: are the 2009 high-intensity discharge headlights the same colour as those from years past. As you can see, it was the sort of conversation only engineers can have 
Afterwards, when I mentioned this conversation to a non-engineer friend of mine (yes, I do have a few of those), he reacted with surprise: “You mean that the turbocharged direct injection engine isn’t only available on the TDI?”
What was obvious for us doesn’t seem to be very clear to everyone. So I’ll try and get things straight. What follows is an outline of carburetion systems found in internal combustion engines.
The air-fuel mixture
In a nutshell, fuel needs to be vaporized for it to easily ignite. Getting the right mix of fuel and air is important for getting the best fuel-efficiency, not to mention reducing emissions and just plain waste. In the pre-electronic age, mixing fuel and air was done by the carburetor. It was a fairly precise piece of equipment, and it got very good results, but it wasn’t perfect.
Fuel injection improved things remarkably. The first systems were mechanical: they were more precise, but still not perfect. Electronic injection came along and improved engine performance: the ideal quantity of fuel could be measured and the minimal quantity injected to match engine demand most situations.
In all three cases, fuel and air are mixed outside the cylinder in the intake manifold, which ensures the mix is uniform before being ignited by the spark plug in the cylinder.
The diesel-air mix
Diesel engines adopt a completely different approach. The mixture always has more air than required: a specific quantity of fuel is injected to match conditions of the moment, while the engine can ingest as much air it wants to take in. Timed injection was thus the only way to get a precise quantity of fuel that is not related to air volume, as in a carburetor : injection is then associated to diesel since the beginnings. The first diesels injected fuel indirectly into a precombustion chamber. Then came direct injection in the cylinder, with an additional device, the turbocompressor, to inject more air into the cylinder. The TDI engine was thus born. Today, electronics play an important role in both diesel and gas engines.
TDI with gas?
Until recently, direct injection was difficult to adapt to gas engines for technical reasons, even though the technology has been around since the 1920s. Today, the difficulties have been overcome, and several car-makers offer gas-engine direct fuel injection for better performance, reduced emissions and better fuel-efficiency.
So direct injection is indeed offered with some gas engines, and the trend will definitely continue to intensify in the coming years.
Furthermore, the turbocompressor that gives air-loving diesels their incredible performance can also be adapted to gas engines. Satisfactory results only came in the 1970s with the arrival of mechanical injection, but today’s high-performance turbo gas engines are everywhere and totally reliable. So, yes, there are turbo gas engines on the market today.
Are there turbo gas engines with direct injection? Once again, the answer is yes. Volkswagen’s 2.0T, called the TSI, offers the best of both worlds. Mazda, GM and Mitsubishi, among others, also offer turbo engines with direct injection. This type of state-of-the-art gas engine was largely made possible by the arrival of TDI technology: it was TDI that solved the main problem of synchronizing the precise moment of injection.
OK, I’ve tried to be brief, perhaps too brief, so if you have any questions, let’s talk.
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