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Name: Daniel
Status: Student
Grade: 9-12
Location: MI
Country: United States
Date: February 2008


Question:
Will higher oxygen level in air fuel mixture make a better burn/combustion? I.E. NOT higher air volume but higher oxygen in air entering internal combustion engine.



Replies:
Hi Daniel,

With no other changes to the engine, a higher oxygen content in air will not result in better combustion. To understand this, you need to understand some details of a modern internal combustion engine. Take a normal car engine for example. Any modern car engine uses a fuel injection system that comes very close to operating with the exact stoichiometric air(oxygen)-fuel mixture. That means that it senses the amount of air the engine "inhales" and injects the nearly perfect amount of fuel to burn all the oxygen in the air. Note that this is based on the air containing a normal 21% oxygen.

If you were somehow able to increase the oxygen content in air, the injection system would have no way to know this, and would continue to inject the same amount of fuel, that it would inject for ordinary air. As a result, the additional amount of oxygen you added to the air, would not have any fuel to burn with it and would simply go out the exhaust pipe. What is worse, the lean (excess oxygen) mixture that will result, will likely result in poor engine response and annoying "drivability" issues that result from an excessively "lean" mixture.

One additional point is that most modern car motors have what is called a "closed loop system". Here, an oxygen sensor is inserted into the exhaust. This feeds back a signal to the injection system to allow fine tuning to get an absolutely perfect air fuel mixture. If you used your "special" oxygen enhanced air, the oxygen sensor would notice that there was some residual oxygen left in the exhaust, and command the injection system to use more fuel to burn with the oxygen. In this case, more power would result. But this would only work for a small increase of oxygen in the air, because this feedback system is only designed to make small corrections. It would not be able to handle a large increase in oxygen in air, so once again, most of the extra oxygen you added would just go out the exhaust.

Note that it is possible for large increases in power to result from a motor that is designed to use pure oxygen, and which is designed to inject the greater amount of fuel to burn with it. Many torpedoes operate this way. Most of a torpedo's body is a large tank of high pressure oxygen, that is fed to an internal combustion motor that drives the propeller. These motors are designed to use pure oxygen and have very high power for their size.

Regards,

Bob Wilson


Probably. It depends on what you mean by "better."

With higher oxygen partial pressure in the air, you can use a smaller volume of air to achieve complete combustion, meaning less inert nitrogen gas. This will make the exhaust temperature higher, which allows for higher thermal efficiency (more work for the same amount of fuel burned). This may, however, be a problem if the higher temperature damages the engine.

With higher oxygen partial pressure in the air, the fuel will complete its combustion sooner. This may be a problem with the engine because the pressure and temperature in the cylinder will rise faster. It would probably require an adjustment in timing to obtain optimal performance, because the highest pressure would occur sooner in the cycle.

If you use the same volume of air, there will be just plain more oxygen in the fuel/air mixture. This will result in more complete combustion of the fuel, which would mean production of less carbon monoxide and fewer unburnt hydrocarbons. The problem here is that it will also result in the production of more nitrogen oxides.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Yes, in principle, assuming the fuel / oxygen ratio is balanced accordingly. This is one of the reasons rockets use liquid oxygen rather than liquid air (about 80 / 20 nitrogen / oxygen) to burn the fuel. However, on the Earth's surface the difficulty of using enriched oxygen is over-weighed by the energy cost of moving the oxygen along with the vehicle.

Vince Calder



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