FSI

No, this posting is not about a secret merger between the American FBI and the Russian FSB. Such a concoction is only plausible in a hell frozen over during the peak of global warming. But this posting is over a merger of sorts, albeit two thermodynamic engine cycles rather than instruments of state power. Indeed this posting picks up on my recently acquired Audi A4, which uses the so-called VW-Audi 2.0 FSI engine.

You may recall my gripes with the new car having some noticeable engine vibration at certain low engine speeds. As it turns out the source of these vibrations may be traced to the nature of the beast, the FSI engine. In essence, the FSI runs on a four stroke cycle that combines elements of the Otto and Diesel thermodynamic cycles. Whereas this engine implements the basic Otto cycle, which requires an explicit ignition system to function, it also uses direct fuel injection, like Diesel engines, and runs a high compression ratio for its configuration.

Those familiar with operating Diesel-powered cars may recall that these engines are prone to emitting strong vibrations, especially at lower engine speeds under power. The cause of these vibrations is rooted in the exceptionally quick burn rates of the fuel mixture during compression ignition inherent to the Diesel cycle. In the FSI engine, a similar if less dramatic phenomenon occurs under power.

Due to the direct injection and a remarkably high compression ratio for a turbo-charged spark ignition engine, the FSI engine must be experiencing significantly higher burn rates of its fuel mixture than is normal for traditional Otto cycle engines. Evidence in support of this notion can be found in the use of a significantly strengthened crankshaft in the FSI engine, according to Audi technical publications on this engine.

The upshot of the FSI concept is that the engine can burn very lean mixtures under light load if it runs on proper, high quality fuel, as can be found in Europe. This makes for a fuel efficient, powerful engine with low emissions. However, in the USA where fuel is of less favourable quality, the FSI concept has to be compromised, again according to Audi literature. In my opinion, the fall-out of all that is an engine that runs somewhat rougher than intended on the diet of available US fuel.

My point is: Why must Audi pass on to the customer the issues resulting from their implementation choices? I do not want to know about vibrations because Audi in all their wisdom and experience has been caught out by sub-standard fuel in their biggest market outside of Europe. And the GTI 2.0T that I drove two years ago and which used the FSI engine, did not display these vibrations as far as I could remember. Clearly the installation in the VW is less susceptible to these issues. And according to road test reports, the A3, which shares the GTI platform is similarly free of these vibrations.

On the flip side, there are some performance figures to suggest that the 2.0T FSI engine in the A3 and GTI are somewhat less tweaked than in the heavier A4, although on paper the power specification is the same across board. If true, it would explain to some extent the increased vibrations in the A4 implementation of the engine, which would be running higher combustion peak pressure and probably a higher burn rate than in the A3. Also, the different installation layouts can explain the difference in perceived vibrations between these two versions of the Audi.

As a footnote it must be said that yours truly is particularly fastidious about these matters. None but one of my friends so far have noticed the vibrations that I have mentioned. One friend, a South African mechanical engineering student currently at MIT, did look rather surprised once when I accelerated the engine through the boom that emits at 2400 rpm. He clearly picked up something out of the ordinary and quietly checked the rev counter to see if I was labouring the engine at too low a speed. Nope, I was not.

I guess all engineering is about compromise, regardless of the marketing blurb. I suppose for me it will be a matter of accepting the status quo for the sake of driving the acclaimed Quattro platform propelled by one of the best new petrol engine designs on the automotive scene. It a pity that somehow it all got a bit spoiled in the implementation and deployment.