What are the drawbacks of using mogas in aircraft?

Answer 1

The drawbacks include the fact that in many cases, you simply can't do it. Mogas has higher flame, flash, and saturation temperatures than 100LL and lower flash, flame, and saturation temperatures than kerosene based fuels, such as Jet A, JP-8, etc.

In the case of an engine designed for 100LL, the fuel would flood the combustion chamber and extinguish any flame that was in there.

In the case of an engine designed for kerosene based fuels, it would likely vapor lock and possibly damage the fuel control unit and other engine components.

However, all this being said, 100LL engines can be modified to take ordinary mogas. The disadvantage of doing this is the fact that mogas is more dense than 100LL. The consequence is that any given specific quantity, say 40 gallons for example, of mogas is going to weigh more than 40 gallons of 100LL. The advantage of doing this is that one can refuel their own airplane at an airport without having to buy fuel from the airport (and aviation fuels are quite expensive).

Answer 2

I ran MOGAS (with an occasional splash or AVGAS) in two C-172s commercially for several years. The only problems I had were scoring on the cylinder walls and one stuck valve. I've seen both of these problems with AVGAS too.

Carb icing and vapor lock are potential problems that I never encountered. The engine manufacturers copped out on their warranty when work was needed because of MOGAS use.

The nice thing about MOGAS is no sparkplug fouling. The plugs were so clean you could eat off of them after 100hrs. With AVGAS (even leaning on the ground and running hot) the bottom plugs are almost filled to the top with lead deposits.

Most of the lower performing engines (like the O-200, O-300, O320) were designed for 80/87 Octane fuel and run good on MOGAS. Be sure the aircraft is approved for it first.

Answer 3

Generally speaking, most piston engines originally designed to run on 80 octane AvGas will run just fine on 87 octane or higher automotive fuel (MoGas), and supplemental type certificates (STCs) are available to certify many small aircraft to be able to use this fuel. There are some concerns about using ethanol blended fuels, but most of these concerns can be dealt with, if your STC allows it.

The biggest drawback to using unleaded MoGas is that in addition to being an octane booster, the lead in AvGas was used to lubricate the surfaces of the intake and exhaust valves. While most newer engines, and those rebuilt with newer components have hardened valve seats to reduce the need for lead in the fuel, many older engines do not. The removal of that lead from the fuel supply in older engines may lead to advanced wear of the valve seats, and to early failure of the engine.

Answer 4

Kelly S is a smart fellow and gave a very good answer. Generally speaking an STC (supplemental type certificate) is available for many general aviation aircraft operating under part 91 of the FAA regulations to use mo-gas however; if the STC is adhered to not all mo-gas is approved and the aircraft may require some modifications to use the fuel because of problems with things like o-ring seals, hoses and so on that would deteriorate from the use of mo-gas. Transportation of fuel from the pumps to the airport to fuel your plane can be dangerous if not done properly. Unlike av gas mo-gas will build fuel varnishes over time that you don't want in your aircraft engine. At higher altitudes mo-gas will not give the same performance as avgas. It is the pilots responsibility to make sure the mo-gas fuel meets the requirements of the STC. Your plane will smell more like a tractor than an airplane.

For my money I burn nothing but 100LL AV GAS!

Answer 5

Several good answers but remember:
1. ensure your engine is approved and use the STC
2. use midgrade (92 octane)or higher gas and filter it when fueling
3, a gravity feed fuel system (c-150,172) works better than fuel pump (low wing systems) due to potential vapor lock problems.
(Vapor lock is when the fuel boils. Oxygen is released and forms an "air pocket" in the fuel lines near the heat source. )

4, Mogas has a lower reid vapor pressure (boiling point) index than avgas. A fuel pump lowers the already low RVP even lower. This can cause vapor lock on hot days especially during a prolonged taxi or while waiting in line for take off. My C-85 did NOT like avgas (the rings kept sticking). I switched back to avagas and no more problems.

Answer 6

Mogas or gasoline used for automobiles has a lower octane rating than avgas, so it can only be used for smaller engines, with less horse power output, that are specifically certified to use mogas.

Also, Avgas still uses led (to increase octane rating) and mogas does not.

But most importantly, mogas is often mixed with ethanol, and this substance can be very harmful to aviation engines, if you are going to use mogas, make sure it is NOT mixed with ethanol.

Answer 7

One other drawback of Mogas/Autogas is that it has a "shelf life". If you don't use it within a few weeks, it starts to go bad. Many people I know who use autogas will only use it if they know they'll be flying for a while... then they fill the tanks with 100LL.

Answer 8

Kelley has got it.

Just to add, mogas is susceptible to carb icing, (not good)
and it is not as clean as avgas.
Mogas can suspend a lot more water in it than avgas.

Avgas is constantly checked for debris in it.