By Ross Elliot
Propulsion technology for the last 80 years has evolved from the internal combustion engines used by the Wrights and early pioneers, to the radials seen in WW1 and WW2, and finally to the horizontally opposed engines we have today. The pros and cons of each of these could be devoted their own article. Surpringly what is old becomes new again, as we comb history and science for better ways to transfer energy. The rigorous demands of aviation engines have guided the design of aircraft engines throughout the last century. These demands apart from automotive applications have for better or worse presented engineers a paradox, making propulsion development even more challenging. These demands have included;
Light Weight- to allow for higher useful loads
Simple– for ease of maintenance and repair, often in remote places and conditions
High Reliability– Engine failure in Aeronautic conditions can be catastrophic
Fuel Efficient– Requirements for Range
Powerful- Automotive engines only use 20% of there HP at highway speeds, Aircraft 55-75% in cruise
That is a tall order for a propulsion technology, partially explaining the reason current GA tech has not changed much in the last 50 years. The “if it isn’t broke dont fix it” has worked well for previous decades. However, with the rise in fuel prices particular for 100LL averaging 6 dollars a gallon. The drive to find more sustainable fuels and efficiencies has increased. In the next few articles we will discuss current problems and technological possibilities for GA propulsion systems. This list is not exhaustive but will provide an exploratory platform into current technology.
Current Tech-
In propulsion, the game is about turning one kind of energy into another – Kinetic Energy in the form of aircraft or vehicle propulsion. The kicker is doing it as efficiently as possible. The ultimate challenges of “10,000 miles on a gallon of gas” is an example of this kind of quest.
Current technology is about 20 to 25 % efficient in transferring stored chemical energy from Gasoline into forward motion.
A Kilo Gram of Automotive gas contains about 46.4 MJ of Energy per Kilogram, and Aviation fuel about 43.1 MJ per KG. Surprisingly Motor Gas is more energy dense than Av Gas, but we will save that discussion for a little bit later. Mostly it has to do with octane required and Detonation experienced at the higher compression ratios without the higher octane fuel.
Out of the 46.4 MJ of Energy in a KG of fuel (with a standard internal combustion engine) we only recover 20 to 25% of that, about 9.3 MJ. The rest, goes to heat in the exhaust, incomplete combustion, noise, friction in the engine, auxiliary units such as fuel pumps and AC units, and to engine block heat. However recovering the additional energy is not always easy. A small amount of the energy theoretically just can’t be used. Just as in a water wheel that captures the motion of the water, if all the energy were extracted there would be no energy to move the water out of the way at the end, thus needing some energy to facilitate the movement of water to continue. Similarly some waste heat is required just to keep the heat flow going.
We can even take the falling water wheel analogy a step further. In an old fashioned water wheel, water at a higher level falls through a wheel to a lower level. The water at a high level has potential energy stored in it that is released as it falls to a lower level. If this energy weren’t extracted we would have a very fast torrent of water as this energy is turned into motion known as kinetic energy (water fall). However as it is extracted by the water wheel or a hydroelectric dam, the energy is used to do work, resulting in water at the bottom of the wheel with little speed. This is very similar with a heat engine like an internal combustion engine, the difference between internal temperature (very hot) and the exhaust temp (not so hot) is the amount of energy extracted from the process. If no energy were extracted the exhaust would be very very hot.
So in the process of trying to satisfy the before mentioned demands the aircraft engine makers have used a couple of tricks. Some resulting in our current situation, which is most aircraft engines being limited to 100LL. Why do we have 100LL? We can thank the demand for high power to low weight ratio, and the need for engine simplicity for this requirment. Octane is really a rating of how well a fuel resists exploding unexpectedly. Any unexpected explosion in an engine is bad, resulting in Detonation. So why do aircraft engines use such high octane, especially when auto gas has more energy per pound (KG).
Interestingly, Aviation Octane used to be higher than 100LL, and was a result of the demands of very high compression engines during WWII. Higher compression engines produced more power per size, which is good for weight sensitive aircraft. However, high compression engines also run hotter (good for efficiency), but when fed low octane fuel mixtures these engines want to not just burn quick, but explode uncontrollably (bad). So fuel manufactures introduced additives to resist this exploding, one of these being a form of Lead. This lead (no pun intended) the way for high compression engines in the aviation world. Giving us high power to low weight required for most good performing aircraft.
As can be found elsewhere main stream fuel went on to different additives due to the health problems associated with Lead. These additives were capable of achieving 87-93 octane but the higher levels were more problematic. Higher Octane fuel thence become more of a niche, thus giving us the higher prices we have today. Our aviation engines have also grown dependant on this lead to protect the internal components of aviation engines. Leaving the industry in a quandary. Most aviation engines can’t run Auto Gas due to relatively high compression compared to cars and the need these engines have for lead to protect some critical components such as valves.
So currently there are a couple of ways designers have tried to make aircraft more efficient. Later we will explore other power-trains for there potential. One way is to reduce weight and increase aerodynamics through the use of composites, thus reducing the size of engine needed. The smaller the engine the smaller the fuel burn. This has resulted in small gains as one can find on the aircraft comparison charts on this site. Another way has been Turbochargers and inter-coolers which have increased some efficiency. However, more often than not turbochargers and inter-coolers are used to boost power instead of efficiency, unless operated at higher altitudes, where they have made one of the largest inroads to increased efficiency of recent developments. The Turbocharges recapture some of that wasted heat in the exhaust and use it to drive compressors that put positive pressure on the cylinders, eliminating some pumping loss used to suck air into the cylinders in a naturally aspiratted engine. This also allows an aircraft to run at higher power settings at altitude, taking adantage of faster true airspeeds. Interestingly the turbine on a turbocharger could be used to increase torque but most often is used to run a compressor. Inter-coolers, are heat exchangers that cool down the heat created by compressing the air in the compressor section of a turbocharger. This cool air is more dense allowing better power. However each of these additions does add complexity that can effect aircraft reliability.
This is the current status of most GA propulsion, the following articles continue interesting insights into possible solutions.
