German Car Zone

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Driven: 2009 Audi A5 2.0T FrontTrak Ethanol Test Mule

Suck a hot colour and the performance figurers are a step up from the 1.8t

Hmmm, finally someone is using a proper engine for E85, all these low-compression engines keep losing huge fuel economy cause E85 is 10 octane. I really hope this makes it into like the A4 and stuff cause the low price of E85 and the low economy loss from a high-compression engine would easily make it a winner.

Saab has been doing that for years. I got to ask you, what do you mean by low compression engines, and E85 has 10 octane? The E85 engines from Saab all runs on a higher compression ratio. A high compression engine doesn't really lowers the fuel economy of the engine, especially with ethanol fuel, since they have less energy per liter or kg than normal petrol.

Haha, sorry I totally missed that I put E85 as 10 octane, I meant to put 105. Wow... I must have been really tired when I posted that.

Well, the thing about a high compression engine is that the higher compression allows you to extract more energy from that fuel so even if the E85 has a lower energy density you might be able to raise the compression high enough and tune the engine for better fuel economy.

True, saab has been doing this for a while, but companies like GM and Ford always end up making low compression engines into flexfuel engines even though it would make more sense to use something like the turbo-4 from the sky since it would be better able to take advantage of the higher octane rating of the fuel so the fuel-economy loss when you use E85 will be much smaller than if you were using something like an impala or a yukon.

Still, I'm happy to see audi using their 2.0TFSI for ethanol, it's a great engine for a knock-resistant fuel like E85. Sorry if I confused you.

Its cool mate, I am very happy to talk to someone who is interested in this kind of thing

Things gets a bit tricky when it comes to explaining high octane and higher compression ratio. I will get back to that when I have a clear head. Engine with a high compression ratio means it can achieve a greater amount of work and it will have has a better thermal efficiency but that doesn't equal to better fuel economy. Basically the engine can achieve a higher output.

I don't know much about Ford and GM and their flex fuel engines but I guess the engine compression ratio can't be too high because the engine still need to run on petrol at part of the time. By the way, what kind of compression ratio do these engines run on?

Yeah, I get what you're saying about a higher thermal efficiency.

Well, the idea of a higher thermal efficiency is that you can lower the fuel ratio so you can extract more power out of that fuel. Ideally, you can get the same power out of much less fuel so you can lower your fuel consumption. Now there's a bunch of other factor that go into this, but this is the basic theory. If they had this E85 engine for the A4 I'd get it as soon as they got the manual trany for it over here in the states.

As for Ford and GM flexfuel vehicles. They're the same compression ratio as every other impala or whatever car they slap the badge onto. All they do is mod the ECU a bit, but from what I've read the fuel economy loss doesn't justify the "fuel savings"

But heck yeah I'm into alternative fuels like this. Hell, I'm waiting for them to start making high performance diesels. I mean I know they have the R10 Le Mans car, but to see it start trickling down to everyday cars that can outperform their petrol counterparts, that'll be the day. Still, E85 isn't a bad fuel if given a good engine to combust it with. After all, who doesn't like gobs and gobs of torque and a nice exhaust note?

Well it is all about balancing each element in the equation,

Engine power = thermal efficiency x equivalence ratio x mass flow rate of fuel x heat energy of fuel

Petrol has a fuel heating value of 44MJ/kg while E85 fuel has 31MJ/kg, roughly 70% of the energy of petrol.

Comparing the engine using petrol and E85, you will need to have a 30%-40% improvement in the engine thermal efficiency to gain the same engine power while keeping the fuel flow rate at the same level. If you reduce the fuel flow rate, you will have a lean combustion process, which will reduce the equivalence ratio, and result in a further loss in engine power. The engine thermal efficiency will have to be further increased to compensate for it. A higher compression ratio will help with the thermal efficiency, but it won't be possible to achieve a 30 to 40% improvement, so they just increase the fuel flow rate, thus highe fuel consumption. At the end, I don't think it is possible to gain the same power while using less fuel.

That sounds like lazy engineering, don't know how they can get away with it.

btw sorry for the late reply I have been pretty busy.

It's cool. I've been really busy myself what with school and finals coming up really soon.

True, an engine running E85 probably can't get the fuel economy of an engine running straight petrol, but they can still try to creep up on the efficiency of the engine. I just hope they start getting into different alternative fuels. I mean, why not? They've already shown that diesel which has been considered a "dirty fuel" over here in the states for years to be a very viable source of fuel for cars just about everywhere else. I remember every time I've been to europe, most of the cars run on diesel. Why? Cause it's cheap to refine, the performance can be about the same as a petrol car, but the fuel economy is just so much better. I really couldn't complain if I had a nice diesel sedan or hatchback over here, but the EPA deems most of those cars unfit (hence we only have like... the jetta tdi and a couple MB's).

And yeah, it is lazy engineering, it kinda saddens me. I remember reading a post, I believe it was on t*rbobricks where a guy was modding one of his cars to run on E85 and he actually took his friend's Impala and looked at the ECU. The fuel mapping for the petrol was pretty dimwitted. As for the fuel map on E85, he basically made it sound like someone had taken a drawing book, taken some crayons, scribbled around, and deemed it fit to be a good ECU.


In any case. I wonder if this'll make its way into the new A4, I'd love to have that engine in that B8.