MadSci Network: Engineering

Re: Why does hydrogen peroxide injection reduce emissions in 2 stroke engines

Date: Wed May 19 12:10:34 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 926974436.Eg


I have read the Mercury Marine web pages at the following URL and I could 
not find any reference to hydrogen peroxide injection into their engines so 
Iíll have to make an educated guess to answer your question:

Combustion, or burning, is a rapid, self-sustaining chemical reaction that 
releases a significant amount of heat. Examples of common combustion 
processes are burning candles, forest fires, log fires, the burning of 
natural gas in home furnaces, and the burning of gasoline in internal 
combustion engines. For combustion to occur, three things must normally be 
present: a fuel, an oxidizer, and an ignition stimulus. (An exception is 
hypergolic combustion used in space vehicles, in which a fuel and an 
oxidizer spontaneously react on contact without the need for an ignition 

Fuels can be solid, liquid, or gas. Examples of solid fuels include wood, 
and coal. Liquid fuels include gasoline and kerosene. Propane and hydrogen 
are examples of gaseous fuels. 

Oxidizers can also be solid (such as ammonium perchlorate, which is used in 
space shuttle booster rockets), liquid (such as hydrogen peroxide), or 
gaseous (such as oxygen). Air, which contains 21% oxygen, is a particularly 
common oxidizer. 

Electrical sparks or glow plugs are examples of an ignition stimulus. 

Letís review the operation of typical 4 stroke and 2 stroke internal 
combustion engines: 

Stroke 1: the oxidizer (air) and fuel (gasoline) are drawn into (or 
injected into) a cylinder during a piston down stroke. 

Stroke 2: The fuel/oxidizer mixture is compressed during a piston up stroke 

Stroke 3 : The spark plug ignites the fuel/oxidizer mixture driving the 
piston down in a power stroke.

Stroke 4: The rising piston drives the combustion by products completely 
out of the cylinder during an upstroke 

The advantages of the 4 stroke engine are that the intake, compression, 
ignition/power and exhaust cycles are separate events that are efficiently 
completed, each having about 25% of the total time of operation.

The disadvantage is that 2 rotations of the crankshaft are required for 
each power stroke. Four stroke engines typically have several cylinders to 
enable a smooth running engine, particularly when operated at low 
revolutions per minute (RPM).

Small, compact, air cooled internal combustion engines often use a 2 stroke 

Stroke 1: On the upstroke a fuel/air mixture is compressed along with some 
residue exhaust gas from the previous cycle.

Stroke 2: The spark plug ignites a fuel/air mixture providing a power 
stroke which also exhausts the combustion products at the end of the 

The advantage of the 2 stroke engine is that a power stroke occurs for 
every rotation of the crank shaft.

The disadvantages are incomplete fuel/oxidizer intake and combustion 
exhaust processes due to the short time (a few % of the total operation) 
available to perform these functions.

To make 2 cycle engines more efficient with less pollutants in the exhaust, 
an optimum fuel/air mixture can be quickly inserted into the cylinder with 
a high pressure fuel injector. This has improved 2 stroke engine power and 

However, because air is composed of 78% nitrogen and only 21% oxygen (the 
oxidizer) the fuel/oxidizer  mixture is still far from optimum, 
particularly with the 78% of unwanted, pollution causing nitrogen in air.

By injecting an efficient, non polluting  oxidizer, such as hydrogen 
peroxide (H2O2), into the cylinder along with the fuel/air mixture, 
additional oxygen is made available for combustion process and water (H2O 
as steam) is a major by product from the added process. The steam can also 
remove heat from the cylinder reducing the need for cooling of the engine.

Hydrogen peroxide could improve the operation of 4 stroke engines but the 
gain is not nearly as great as would be obtained with 2 stroke engines. 
Here in California many industries cannot not use air in there combustion 
processes and liquid or gaseous oxygen are used in place of air to greatly 
reduce air polution. However, these are typically fixed sites and not 
vehicles. Hydrogen fueled vehicles are also being tested in many states.

Hydrogen peroxide (H2O2) is an interesting chemical that is now being used 
to clean up a number of polluting processes here on planet earth. You can 
read about some of these applications at the following web site:

Best regards, Your Mad Scientist
Adrian Popa

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