As I mentioned it’s the 2008 NAIAS here in Detroit and “being green” is one of the biggest things here. Everyone is showing off their alternative-fuel vehicles. Besides all the hybrids (that’s a topic worthy of a post all its own) and the smattering of plug-ins and hydrogen fuel cell cars, E85 seems to dominate when talking about “getting environmentally friendly.” Heck, even the racing segment is getting into the mix with the Corvette C6.4 GT1 team switching to E85 for the 2008 season. (Side note: Compuware is the team sponsor and despite my dislike of E85 that car is sweet.)

Despite it’s seemingly unstoppable popularity, our current approach to E85 ethanol based fuel isn’t really the best idea in my opinion. It’s not that I like being tied to petroleum based gasoline or think that research into alternative propulsion systems is a bad idea; far from it. It’s just that I really like popcorn… and that E85 is more like a stopgap measure that isn’t even all that effective. If it is created from corn then the total energy life cycle of E85 is actually less efficient than producing gasoline from oil. Hard to believe? This three part post will help to inform, I hope.

Now, being the son of an engineer who worked at FoMoCo for over 40 years (much of it in powertrain R&D) and of course being a native son of Detroit this kind of car stuff is in my blood, so allow me to indulge for a bit in part one of this post and explain first a bit about the engineering principles at work here. I will assume you don’t know much about internal combustion engines but if you do feel free to fact check me and please don’t feel like I’m talking down to you.

Alcohol based fuel itself isn’t actually so bad on its own. On the contrary, E85 is in many ways better than petroleum based gasoline. One of the major pluses is that it has a octane rating more than 10 points higher than that of premium unleaded gas. And while the energy density of an ethanol based fuel like E85 is actually lower than gasoline, the fact that it’s octane rating is so much higher means that you can run it in engines that have higher compression ratios (12:1 or even upwards of 14:1, compared to a somewhat typical 8:1 on a normal car that can run on regular gas) without risking the dreaded pre-detonation (not knock, which is different).

In case you don’t know, pre-detonation is basically when the fuel explodes due to compression in the cylinder pre-spark, which is very bad in a typical non-diesel engine. At the very least you’re ruining your efficiency and more typically you’re causing physical damage to the engine itself. My dad tells me it’s so bad that once you get pre-det occurring a hole will form in the piston head within seconds. The reason the fuel explodes early is because hot spots in the cylinder (on the edges, carbon buildups or other protrusions) cause the mixture to ignite before the spark. Since your explosion happens at the wrong time not only do you lose a lot of power, but the heat builds up in the cylinder because it isn’t being exhausted properly, and the condition spirals out of control. Explosions start happening sooner and sooner until it causes cataclysmic engine failure. In short, you don’t want this. It will destroy your engine.

Knock on the other hand, is when you have multiple explosions in the same cylinder, one or more not resulting from the spark. The trick to get optimal power is to spark at the right time (earlier in the cycle) so that when the flame front has fully propagated through the cylinder the mixture reaches its optimal compression. They do this through spark advance, igniting the spark at some point before top dead center (TDC). If all goes well the explosion is even and you reach peak pressure about 15 degrees after TDC. This gives you optimal power output (and therefore efficiency).

But what happens if the mixture ignites too early, and the resulting flame front causes a pressure wave that ignites the mixture at the back of the cylinder, causing a secondary explosion? What if the radiated heat from that explosion causes another pocket of mixture to explode? When you get these multiple flame fronts colliding this causes the cylinder pressure to go far above what it’s supposed to be, causing knock, or detonation. It’s not quite as bad as pre-det, but still pretty bad. If your engine is knocking really badly, it can actually cause stress fractures in the cylinder, dent the piston and even bend the valves. Some cars will detect knock and dynamically retard the spark timing (effectively retuning the engine for lower performance and efficiency), thus preventing detonation but also reducing your economy and power. I guess that’s still better than destroying your engine.

Not to go off on too much of a tangent, but this is why you need to run premium gas if your car has a higher compression ratio, a trend that is becoming much more popular. One way of improving fuel efficiency is to get more energy out of every stroke, hence slightly higher compression ratios (9 or 10 to 1). Turbos and superchargers can help a bit more, improving the power output (and therefore fuel efficiency) by increasing pressure on the intake manifold, thus allowing more air and fuel to enter the cylinder. But more boost means higher pressure, which means you need a higher octane fuel to avoid pre-det. I could go on and on about all the complexities of turbos, intercoolers, air/fuel ratios, waste heat, advances in direct injection… but I’ll save that for another post.

Despite all the stuff I’ve written so far, I suppose I haven’t really explained why a higher compression ratio is so good. Here’s the simple answer: Higher compression ratios give you more energy per stroke because the air/fuel mixture has a higher thermal efficiency. Assuming the fuel is well vaporized in the cylinder this higher compression means a more even detonation and burn, extracting more energy from the tiny droplets of fuel as they burn with the highly compressed oxygen. This means you achieve peak pressure sooner, which results in more mechanical work performed. In simpler terms, bigger boom means harder push on the piston which means more power as well as better emissions because more of the fuel has burned by the time it hits the exhaust port.

So to get back to what is actually good about E85, higher compression ratios mean the fact that the energy density of ethanol is only 30 MJ/kg - while gasoline is nearly 47 MJ/kg - isn’t a problem. But that’s not the whole story. The truth is that ethanol needs to be run at a much richer mixture than gasoline, that is, you need more fuel for the same volume of air. Specific amounts depend on the engine of course, but the stoichiometric values (optimal air/fuel mix) are 14.7:1 for gasoline and 9:1 for ethanol (slightly higher for E85).

(By the way, hopefully now that I’ve covered all this you can start to appreciate how amazingly hard it is to calibrate an engine correctly for use in a wide variety of temperatures, different throttle positions, different kinds of fuels and allow it to remain calibrated over a long period of time. Yeah, it’s complex.)

In an engine that is designed to run nothing but ethanol (or E85) all of this can be overcome. High enough compression ratios and careful tuning mean that you can extract more energy from ethanol, resulting in better performance, better efficiency, and lower emissions.

The problem is that there are hardly any vehicles a consumer can buy that are specifically made for running on E85, they’re almost all “flex-fuel” vehicles that can run on gas (regular gas mind you) or E85. And so we come to our first reason why E85 the way we’re implementing it is a bad idea. But this post has gone on long enough, so I’ll save my specific criticisms for part two.

Edit: My dad was good enough to read this and send me a few comments. As such, I’ve edited the post a little bit. You might want to re-read the section about knock (or pre-detonation, which is what I’m actually talking about it seems). Basically pre-detonation is independent of knock, but pre-det CAN cause knock, and knock CAN cause pre-det. It’s confusing, I know. Thanks dad!

Edit: Part two is up.

This entry was posted on Thursday, January 17th, 2008 at 11:57 am and is filed under Anything and tagged with cars, resources. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.