Posted on 02/21/2002 1:31:00 PM PST by John Jamieson
Hi Efficiency Engine Design
John Jamieson MIT67
Ive been studying the reasons that modern internal combustion engines operate at 25 to 30% efficiency for the last year or so. There are basically three main areas that seem repairable but would lead to large efficiency increases.
1. Current engines are symmetrical stroke. They have the same compression ratio as exhaust ratio. About 10 to 1 is the limit for compression ratio with modern gasolines, but the ideal expansion ratio is more like 25 to 1. Atkinson realized this problem in 1896 and patented and built many engines to prove the concept. He was run over by lighter, smaller, cheaper Otto cycle engines. (Current Atkinson cycle engines are really Miller cycle engines, without asymmetrical strokes).
2. Current engines cannot adapt to variable displacement to adjust output. High output can very efficient but low output requires throttling of the air, reducing compression ratio and efficiency. (Most cars only require 10 to 20 horsepower to cruise at 60 mph).
3. Current engines generate about half their internal friction due to piston side loads. Several patents claim to correct this but most are statically indeterminate, which means they dont work.
Come up with a new engine design that fixes these three problems and youll improve IC engine efficiency by 50 to 100%.
Please dont tell me about any existing technology, Im familiar with the vast majority of existing designs, having studied over a thousand patents. Most dont close to solving these three problems in any practical way. Original ideas only please. Yes, I do have a design that solves two of the three problems, that Id be glad to share with anyone interested. (Graphical simulation in VB available).
But that would be really dumb to build a multi-cylinder engine where all cylinders fired together.
The harley people would love it. Except their girlfriends would lube up the seat so bad they would slide right off.
It works out that compression ratio is the main parameter that determines efficiency. There are gains that can be realised by making sure ALL the induced fuel gets burned (more turbulance, residence time, etc.) but this only goes so far. In order to increase the efficiency of any thermodynamic cycle, heat has to be added AT THE HIGHEST TEMPERATURE of the cycle.
Any hydrocarbon fuel has about 20,000btu/lb, whether it's natural gas or crude oil, so that's all we have to work with. With higher compression ratio, higher combustion temperatures are accomplished and benefit efficiency right up to the metallurgical and mechanical limit of the engine.
So a diesel engine with it's 21:1 ratio (it wont even run with less than about a 16:1 ratio) will always be more efficient.
Higher pressures also produce more nitric oxides and thus are not allowed leading to:
more unburned hydrocarbons and carbon monoxide in the exhaust leading to:
a post drive train burner (catalytic convertor) needing:
more expensive gasoline without lead, sulphur, phosphorous leading to:
more crude oil per gallon of gas.
The additional discharge of heat into the manifold leads to heavier radiators needing heavier engines. I'm guessing that the cost of complying with the nitrogen oxide regulations costs about 35% in efficiency, not to mention that photo-reactive partially burned hydrocarbons are still increased.
The issue is that the fuel to air mixture is still not optimized at all times when the car is throttled at different levels. The ideal ratio of air to gasoline is 14 to 1 (by volume) for ideal combustion. To my understanding, the fuel injection systems of cars today still do not overcome this in any major way, and that is the main issue in poor efficiency of the overall system.
You can mix the fuel and air much more efficiently at a higher temperature but then the fuel mixture tends to become stratified and the result is severe detonation in the combustion chamber. Designing a way to overcome this detonation issue by heating the mixture but mixing it in a more uniform way may overcome this.
Popular Mechanics did an interesting series on this subject back in the early 80's. There were some guys who build race car engines who were experimenting around with trying to get the air-fuel ratio improved by heating and mixing the fuel.
I knew a guy who received his engineering degree at General Motors Institute. From my discussions with him, I decided that they didn't teach people to think outside the box at GM, they just wanted people to make small improvements and continue the basic designs that they have been working on for years. I would be surprised if they ever overcome this lack of efficiency in the basic internal combustion engine at any major American car manufacturer.
Solve the engine efficiency problem and you will absolutely change the whole world. And I firmly believe that someday someone will. Might as well be you!
Good hunting!!
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.