Aircraft Powertrains

With the rise in fuel prices, large and small companies alike are looking into new innovative ways to power automotive and aeronautical vehicles. The current trend has been hybrid power-trains, many of the innovative power-trains have focused on taking chemical energy contained in fuel and turning it into propulsion energy.

This article will take a look at the basic principles behind these ideas, and each of thier pro's and cons.

We need to lay a little ground work about the basic propulsion tech we already have.

Currently internal combustion engines used by transportation, turn about 20 to 25% of the chemical energy stored in the fuel into usable forward propulsion. The rest of the energy as other articles have already discussed have gone toward waste heat, noise, etc. This will help lay the framework of the practicality of other propulsion technology.

The Hybrid power-train-

Engineers recognize that an internal combustion engine is not always 20 to 25% efficient. If the car is not going anywhere, then we can reasonably believe that an idling engine is 0% efficient. On the scale of rpm an engine runs, there is a sweet spot where the engine is most efficient at producing power for the least amount of energy. This sweet spot occurs around 40-50% of red line, which is about 70-80% of torque. The sweet spot is bounded by two sides of inefficiency, above 50% red line the engine components experience additional friction due to the faster motion and require more fuel to achieve the required power, reducing efficiency. On the lower side of this sweet spot the engine loses efficiency through “pumping losses”, this occurs as the throttle body is closed to reduce airflow and power, thus increasing the amount of drag the engine feels trying to suck air through a smaller straw. At about 40-50 % rpm the engine gets the largest “bang for the buck” so to speak. Unfortunately, in a standard car or aircraft we require a variable amount of power depending on the circumstance. Making our ability to always run at this sweet spot difficult.

The wide range of power circumstances also necessitates a larger engine to cover these requirements. However we rarely ever use all the power that is available, and normally run most of the time in a low power range. Engine designers take this into effect in optimizing engine power for application. But, the designers still have to design engines for potential cases that might require additional power, such as in acceleration, go around, or for takeoff. The need for a larger engine adds weight which requires additional power. Weight and cubic displacement that we rarley use. In automotive applications only 20% available horsepower is in use at highway speeds. I have wondered how close aircraft engines run to their sweetspot (and would love to ask a Lycoming or Continental Engineer), since thier RPM's are rated at considerably less than standard automotive engines. Perhaps that is a question for another day.

Suffice it to say if there was a way to reduce engine size to exactly what we needed to to run 40-50% rpm almost all the time, that would be a great reduction in weight and fuel. Two interrelated benefits.

The hybrid power-train is the solution automotive applications have been trying . The hybrid power-train reduces the engine size to just barely the hp needed to run 40-50% rpm most of the time and produce all the “average” power needed. This is true for both a parrellel and series hybrid. A series hybrid instead of using the engine to drive the wheels, it drives a generator. The car then uses the electricity from the battery, and sometimes from an engine in high power situations to run an electric motor. The motor runs at close to 95% efficiency at almost all torque requirements. It does a very good job of turning electricity into motion. The only drawback is the energy loss that happens at each transition, IE chemical to heat to motion, motion to electricity, so forth. To be able to see this in comparison lets look at a couple of examples.

Standard Power-trains

A KG of gasoline has about 46.4 MJ of energy.

Gasoline 46.4 ---->standard 4 stroke engine 20% efficient (46.4*.2) ----> Work output 9.28 MJ

Lets compare with the Hybrid.

Gasoline 46.4 -->small constant speed 4 stroke 40% efficient (46.4*.4)-->18.56MJ crank Motion--> Generator 80% efficient (18.56*.8)-->14.85 MJ Electricity--> battery bank 80 % efficient (14.85MJ*.8) -->11.88--> electric motor 95% efficient (11.88*.95)--> Work output 11.28 MJ

The non hybrid turned about 20% of the chemical energy into work, and the hybrid turned about 24% of the chemical energy into work. To turn this into something meaningful, it is the equivalent of a 300 mile tank now getting an extra 64 free miles for the same amount of fuel. That is just over a 20% gain in fuel efficiency just by leveraging strengths of different energy systems, and using them where thier more effective.

This efficiency could be driven up slightly higher if the electricity went straight from the generator to the motor without suffering the energy loss of going to the battery. Then sending extra electricity (created at the constant 40-50% rpm) to the battery and using this battery power to supplement power when the motor demands more than the generator can provide. An energy flow map for the energy moving directly from the generator to the motor would look something like this.

Gasoline 46.4 -->small constant speed 4 stroke 40% efficient (46.4*.4)-->18.56MJ crank Motion--> Generator 80% efficient (18.56*.8)-->14.85 MJ Electricity--> electric motor 95% efficient (14.85*.95)--> Work output 14.1 MJ

This energy flow recovers up to 30% of the initial chemical energy. Translating into 158 free miles on a standard 300 mile tank which is roughly a 50% improvement. Obviously any system combining these two flows will find its efficiency somewhere between them, depending how heavily vehicle depends on one or the other. So it should come as little surprise most of the hybrid cars on the market today do exactly this, they boost there mileage vs their non hybrid equivalents by about 35-40%.

One downside is increased complexity and weight, which in an aircraft system can become a burden, effecting reliability and cost. However reducing fuel burn from 10gph to 6 might be attractive enough.