By Clint Lowe
With avgas exceeding $3 per gallon at a growing number of airports and fuel at gas stations down the street for as little as a third of the going avgas rate, its no wonder that the avgas vs. mogas debate is one that gets new life any time fuel prices climb.
While proponents of mogas cite several advantages it has over 100LL, theres no doubt the biggest appeal comes from its far cheaper cost. But like any operating procedure with two camps, there are plusses and minuses that turn a black-and-white decision into a solid shade of gray.
Because early aircraft engines were based on the automotive engines of the time, automotive fuels were used until engine designers began ratcheting up the power they demanded from the engine without increasing the weight and size of the powerplant. These higher-powered designs overtaxed the anti-knock capabilities of auto fuels of the day. A military-driven effort to improve anti-knock characteristics in fuels saw the first development of an octane specification in 1930 when Fighting Grade aviation fuel became specified at 87 octane.
The octane demands skyrocketed in the years just prior to and during WWII, when the gowling piston engines of front-line fighters started pumping out multi-thousand horsepower – and demanding octane ratings as high as 115/145. To put it in perspective, the World War I-era Liberty engine and World War IIs Rolls Royce Merlin of P-51 fame both displaced 1650 cubic inches, yet the Liberty produced 400 hp on 50 to 70 octane fuel and the Rolls produced up to 2,200 hp on 115/145 octane fuel.
Todays available fuel and pricing has largely been driven by the changes in the aviation environment over the past 50 years. The three major grades of aviation fuel of the 1940s and 1950s was largely due to the volume of fuel being produced. In 1944, when 115/145 was introduced, the war effort saw avgas production hit 25 million gallons a day. This rapidly dropped off after the war to around 5 million gallons a day and, with increases in commercial airline demands, eventually recovered to around 14 million gallons a day in 1957.
As turbines replaced pistons in airlines through the 1960s, demand receded to 5 million gallons a day by 1970 and has diminished steadily to 800,000 gallons per day now. This seems rather puny compared with the current daily turbine fuel consumption rate of 70 million gallons a day and really seems miniscule when one considers the average daily consumption rate of automotive gas in 1999 was around 360 million gallons per day.
With the reduction in demand for avgas, pure economics drove the industry to resort to a single, universal fuel for piston aircraft. Had it not been for the lead-fouling problems Grade 100 had on lower-octane engines, we may well have seen this as that universal fuel. To minimize this problem, 100LL was introduced and has now become the standard aviation gas available nationwide. While there are pockets here and there where you can find the red 80/87 octane and the green 100 octane, they are rare and continuing to disappear.
Lead or No Lead
Engine knock, also known as detonation, is best described as the spontaneous ignition of the fuel/air mixture prematurely. Without getting into a lengthy discussion on knocking, the desirable, smooth propagation of the flame front from the spark plug across the combustion chamber gets replaced with a spontaneous explosion that rapidly raises combustion chamber pressures, having the effect of hammering the rising piston that is still trying to complete the compression stroke.
Continued detonation can quickly damage pistons, rods, crankshafts and the bearings sandwiched in between them. Engine knock is controlled by the fuels ability to resist detonation. This ability is measured as an octane rating.
The octane rating of a fuel can be adjusted through many means. Refiners use various components of crude oil to control octane during the refinement process. Alcohol, when added to gas, raises the anti-knock qualities of the fuel its in. In World War II, emergency war power activated an alcohol/water injection system that effectively raised the fuels octane rating beyond 175.
There are trade-offs to raising the octane level. Refining crude into a higher-octane fuel is more expensive and you can see this at the pump when you compare Premium grade to Regular grade. Alcohol and other oxygenates have lower heat output than gasoline, typically reducing the Btu output of the fuel at least 2-3 percent, with a corresponding reduction in gas mileage (or increased fuel flows for aviators).
One compound with a good balance of cost vs. performance is tetraethyl lead (TEL). You may remember when the term ethyl was used for premium grade auto gas. When first used, regular gas was unleaded and premium used TEL to raise the octane. TEL effectively interrupts the complex chain reactions leading to detonation.
There is a problem with TEL, though. When TEL is burned, it forms lead oxide, which likes to stick to things such as valves, pistons and spark plugs. Without some means of controlling it, these lead oxide deposits would quickly become thick enough to damage your engine.
To control this, a chemical called ethylene dibromide is added along with the TEL. Ethylene dibromide acts as a scavenger by combining with the lead oxide, converting it to lead bromide and lead oxybromides. The converted lead oxide passes out the exhaust pipe. Some of the lead oxide doesnt complete the reaction, though, and small amounts of lead oxide can still be found in the engine over time.
Naturally, the more TEL in the fuel, the more un-reacted lead oxide gets deposited in your engine. Engines designed for 100 octane fuel dont have a problem, but 80 octane engines running on higher levels of TEL than they were designed for over long periods of time accumulate deposits of lead oxide that lead to fouling spark plugs and lead deposits on exhaust valve stems and seats. And the lead difference is significant.
Lead in 80 octane fuel is limited to about 0.5 ml/gal, 100LL is 2 ml/gal and 100 octane is 4 ml/gal – eight times as much lead as is permitted in 80 octane. And herein lies the second big reason many argue for unleaded automotive gas.
Anyone ever running 100LL through a small Continental like the Cessna 150s O-200 has probably run across lead fouling. It appears as rough-running even right in the chocks. Pull the plugs and youll see small globules of lead oxide wedged against the plug electrode, shorting it out. These deposits can be a chore to remove and, at best, needlessly delay an afternoon joyride.
The lead problem in low-octane recips using leaded gas goes way back to the 1940s. Most small engines of that time were designed before TEL was being added to raise octane and, when TEL became the norm, lead problems on valve stems and the like forced manufacturers to re-design the valves to accommodate lead.
This worked well until the engine manufacturers started getting complaints about breaking in new valves – such as following a cylinder swap or after a major/top overhaul – on the unleaded gas that was still available at the time. Since the new valves now apparently needed some lead to prevent overheating and welding of the valves in the guides and seats, in November 1946 Continental issued a service bulletin recommending that engines with new valves should be run two to three hours on leaded gas prior to operating on unleaded. If break-in were performed in this manner, the valves should operate satisfactorily on unleaded for the remainder of its life.
Realities of Mogas
While there are some disadvantages to using auto fuel in engines for which an appropriate STC is available, there are also distinct plusses. I used auto gas successfully in both my 1969 Cessna 150 as well as my 1959 Cessna 172 for several years without any fuel-related problems. I believe the key to that success was in following the guidelines of the STC as well as being aware of the limitations involved with using automotive fuel.
Personally, I observed a number of practices while using autogas:
1. Avgas blending. The circa-1987 Peterson STC called for blending the mogas with avgas in stated amounts. Lets face it, trying to maintain a given percentage of each in a tank with any accuracy is at best troublesome; dead-on accuracy is nearly impossible. I tended to blend on the conservative side, especially during the summer, by using more 100LL than recommended. My rationale in the summer was to go heavier on the 100LL on really hot days to counter any tendency the mogas might have to vapor lock.
2. Buy from a reputable supply. I used gas cans to fill my aircraft tanks, carting the tanks from a nearby gas station. I always I bought from a busy, reputable station where they had the pumps clearly marked with ethanol blends and pure gas, even if it meant paying a few more cents per hour. I reasoned a busy station wouldnt have old gas in the tanks and, with a good reputation, the stuff coming out of the pump was what they advertised and probably not bought at an auction somewhere.
3. Gas cans. First, I never used my airplane gas cans for anything but gas for the airplane. Before ever putting gas in the new cans, Id drill a hole and run a screw through the filler neck (using a small rubber o-ring under the screw head to prevent leaks) so it poked out long enough to get a ground wire to clip to it. I had a long ground wire thatd clip to the exhaust pipe and clip to the can before I began filling the tank.
4. Use the gas. Lets face it, auto gas is poorer quality gas than avgas. It is seasonally re-formulated. It doesnt have to meet the high standards of quality control placed on avgas. If it sits for months in your gas tank, theres no telling what kinds of mischief it might cause. You can be assured avgas will not break down over long periods of time in your fuel tanks or that it wont start eating parts of your fuel system.
5. Store your aircraft with avgas. If, like in North Dakota, recreational flying virtually stops for the winter months, youll find the previous paragraph especially important. As the cold weather sets in, it makes sense to fly off the auto gas and fill the tanks with the 100LL.
Then, after filling it with avgas, fly the airplane to ensure the 100LL gets thoroughly into the fuel system. As in-frequent as flying is during the winter months in most northern states, just running 100LL would probably be the best policy and wont cut into the budget very much.
6. Valve break-in. The EAA (which owns several STCs for autogas) encourages individuals to initially run in an engine with new valves using leaded avgas for at least 10 hours before starting operation on auto gas. I used this procedure on a break-in and never had any problems with the engine.
7. High altitude operations. Though my 150 or 172 didnt go high enough for this to be a consideration, those having aircraft with high service ceilings may want to consider that the higher vapor pressure of auto gas may cause you vapor locking problems at very high altitudes where the atmospheric pressure is considerably lower. If anticipating such operations, it would be wise to put some avgas in the tanks.
On using 100LL
Although auto fuel mitigates some of the problems caused by using purely 100LL in engines designed for 80 octane, there are ways to minimize the lead fouling that plagues these engines.
The principal time lead fouling manifests itself is when the engine is at idle and when its cold. At low RPM, there is insufficient turbulence being generated within the engine cylinders to push the lead oxide out of the engine. This isnt such a problem when the power is up. So, if turbulence isnt available, minimize the amount of lead being generated within the cylinder by minimizing the lead intake.
To do this lean the engine very aggressively immediately after start. Pull back on the mixture until the RPM rises and then take it even a little beyond. You cant hurt the engine at idle power by doing this, but youll be reducing the fuel flow (and lead intake) to the absolute minimum required to get it to run.
After taxiing to the run-up area, advance the mixture and perform the mag check. You can tell youve leaned aggressively enough after start if you try to make run-up RPM and the engine wont do it.
Beyond taxiing, 100LL shouldnt have much of an effect on your engine at cruise RPMs if recommended leaning is accomplished. The high temps and high RPM should see little accumulation of lead in the cylinders. Upon landing, you should again lean the engine for the taxi back to the chocks.
In the final analysis, the evidence appears to show that in many respects operating with 100LL or auto fuel is comparable. So, whats better?
From a personal standpoint, I never found any evidence the auto gas was causing any problems within my engine. The ever-constant lead fouling Id experienced with 100LL disappeared immediately. I overhauled the 150s engine after a couple years of operation on auto gas and could find nothing inside indicating any problems caused by the fuel.
I took the precautions listed earlier and complied with the STC. It worked for me. And, with the amount of flying I was doing, the auto gas saved me huge amounts of money in fuel costs.
On the other hand, anyone contemplating the use of auto gas should be aware that the practice is frowned on by the oil companies and the engine manufacturers, even though its gotten tacit approval of the FAA through STCs held by Peterson and the EAA.
A cynic might say the oil companies are trying to protect their bottom line by selling more high-price avgas, but that ignores the fact that avgas is such a small-volume specialty fuel theyd probably welcome its demise. A skeptic might say the engine manufacturers are trying to shield themselves from potential liability brought on by the vagaries of a less-regulated fuel than avgas.
Both of these positions may have some merit. The truth is that auto fuel in modern engines and airframes just hasnt been studied stringently enough to satisfy everyone. There is anecdotal evidence to support just about any position you want to take on this matter.
As an auto fuel user for years until I moved to an aircraft that cannot use auto fuel, I have a realistic view of what it can and cant do. I came to the conclusion auto gas fall short of avgas in every category except cost and problems stemming from lead depositing. Those might be big enough advantages to outweigh the uncertainties, but then again, they might not.
Clint Lowe is an A&P mechanic who operates a cargo charter operation.