Originally Posted by
mshunter
Very well writen, but I would not consider 8.5 to 1 compression ratios (i.e. IO-550) to be high compression these days.
It seems like a contradiction that aircraft engines have lower compression ratios than the average car. The numbers may differ, but so do the operation regimes. Aircraft engines operate at a higher percentage of their total operating range. Airplane at cruise= 85% or more of total RPM range, versus a car cruising down the highway may be at about 35%.
The main reason why we use leaded gas in aviation is because of it's "cushoining" effects and cooling effects.
Agreed.
Being a former Auto tech, the majority of engines produced today are 9.0 to 1 and higher. Motorcycle engine are 12.5to1 and higher. They all can run on 91 octane. The auto makers have figured out how to make power with unleaded fuels…
Same story here- these engines do not operate at a very high percent of their respective RPM ranges. In addition, the higher compressions you see today are the result of lots of R&D. Back in 1971 when automakers backed off compressions for the first time, values around 8-9 were common. Only later was it possible to bump it back up as engines were redesigned, something which was very expensive to do.
…and higher compressions mostly with valve timing. If you can get the valve timing events right, you can make just as much power on lower octane ratings, with higher compressions.
Agreed- and this is the operating principle behind PRISM (pressure-reactive intelligent spark management system) from GAMI of Ada, Oklahoma. It is not in use due to high certification costs.
Also, aircolled engines tend to operate at higher tempatures than liquid cooled engines.
Agreed in general, especially for cylinder head temps. I will have to look into the flame temps and exhaust gas temps but I think they are similar for the two engines. Cooler cylinder walls can affect the burn quality for the better. A Continental O-520 may run at about 1400F EGT and 400F cylinder wall temps at full thrust. The cylinder wall temps are a lot more than in a water cooled engine. I do not think the flame temps are very different however. Higher cylinder walls would seem to indicate higher octane fuels are needed, because there is more available activation energy (heat).
Have you ever heard an auto "diesel" when it shuts down? This is basically due to the fact that either the timing was to high causing excessie heat in the cumbustion chamber, carbon on top of the piston and in the cyl. heads cumbustion chamber being red hot and continueing cumbustion, or the temp of the engine being to high, causing again, the cumbustion chamber to be to hot and not stoping the ignition cycle.
Yeah I have heard of it. Aviation engines will do it a little bit too even with low lead gas.
On to the catalic convertor issue. Don't run leaded gas in anything with a cat convertor. Like you have stated, it will ruin the convertors efficency, but the EPA is misguided. Remember what the lead does to the valves and seats? It cools them. So it does the same thing to the convertor. A cool convertor works less efficently than a hot one. But what it also does is plug it up. A pluged convertor simply won't let the exhaust flow.
Ok. I am not sure about the EPA being misguided... but it doesn’t matter to aviation.
Now, my opnion on a need for alternative aviation fuel. It simply is not needed. The lead 100LL fuel accounts for in the atmosphere is less than 1%.
EPA agrees as well, hence the exemption we have for aviation using leaded fuels past the 1996 deadline for cars and trucks. But it’s not just about the EPA. Leaded gas is only made by one small company and they could go out of business at any time. Or the EPA could rescind our exemption on leaded avgas. We need to get another fuel ready.
But, if they want to make it work, it can be done without completely re-enginering aviation engines.
Well, 70% of them do not need re-engineering to work to run on lower octanes, but the other 30% do and they are the root of the problem. And the cost of re-engineering them would be very high. Who is going to pay for it? They do not sell enough to spread the cost out and they are already very expensive engines as they are.
So what happened to all the auto's with the improper valves and valve seats. Some of them are still on the road.
And most of them are not. So you are saying there was never a need for re-engineering auto engines to run on no-lead gasoline? You are not correct. There was, and it was done at a cost of many millions by US automakers in the 1970s.
What about 100 (green fuel). It contains no lead, and we can still run it in our airplanes with no ill affects. It's a simple matter of octane.
No- it has even more lead. Avgas 100 Green has about .85 grams per liter lead, while Avgas 100LL Blue has about .56 grams per liter of lead.
What exactly is the affect of higher octane? In it's simplest terms, the higher the octane, the slower the burn. The higher the cyl. pressure (i.e. compression ratio), the faster the burn. So a compression ratio of 10to1 will require a higher octane, or if turbocharged (increased cyl. pressure and heat) will reauire a higher octane for a given compression ratio (8to1 non-turbo=87 octane, 8to1 turbo=93octane). The more pressure an engine makes with the cumbustion process, the more heat it makes, which we have already established will lead to detonation.
Agreed as long as we are speaking in generalities. It’s more about the evenness of the flamefront during the burn cycle but close enough.
If we increase the octane rating of the fuel (i.e. go to 130 octane), we should not need the lead in the fuel any longer.
Actually octane is not the problem. There are many fuel additives that can provide high enough octane numbers, ethanol for example. But all of them have problems such as cooling properties, toxicity to the environment, cost per gallon, low energy density content, and so on. If it were simple problem to develop a replacement for TEL (lead) a major petroleum manufacturer would have done it decades ago.
Take the lead out, the fuel gets less expensive. Increase the octane, it gets more expensive. Back to square one with price now.
I am not sure what your point is here?
In my opnion, there is too much thought going into our fuel, when the fix is rather simple. Increase the octane that is availible(which will cool things down), take the lead out, and the problem solved....It's late, and I have much more to type, but it's time to stare at the backside of my eyelids.
Taking the lead out will destroy 30% of the piston engine aviation fleet, which may seem to be a good solution when you're at the end of a 4 day trip.
This exact strategy used by Detroit in the 1970s to dispose of most of the existing cars that ran on leaded gas in previous ages. But do you as the owner of a brand new $50,000 Lycoming TIO540 airplane want the engine destroyed in say, five or ten years while the rest of the airplane is perfectly fine? Of course not, and this is why there is a big fuss being made over finding a "drop in" replacement for leaded avgas.