Re: How is the physics of the internal combustion engine best described

Greetings:
Heat engines are designed for the purpose of converting heat energy to work.
In the science of thermodynamics there are a number of different models of
heat engines. The ideal heat engine model that best describes the four- stroke
Internal Combustion Engine (ICE) is the Otto Cycle. It is named
after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in
Germany.

From the information in your question I will assume that you are familiar with
the four-strokes of the ICE (intake, compression, power, exhaust). The operation
of this type engine is described and animated on the How Stuff Works web
site:

http://howstuffworks.lycos.com/sc-engines- automotive.htm

Analyzing the mathematical thermodynamic models for heat engines is quite complex
and much to complicated for your project. To get around this problem I will present
some of the results of the Otto Cycle model which you will be able to graph and
discuss the engine efficiency in terms of temperatures. One of the key graphs in
studying the thermodynamic behavior of heat engines is the Cylinder Pressure
versus Cylinder Volume (P/V) curves .
I will base my discussion on the PV curves presented on the
HyperPhysics web site of the Georgia State University , U.S.A. located at:

http://230nsc1.phy- astr.gsu.edu/hbase/thermo/heaeng.html#c1

When you reach the Heat Engine page click on automobile engines and you
will reach the Otto Cycle page. This page has an animated P/V graph which presents
each of the four strokes of the piston in an ICE. This web site is presented for
university students; however, note the number at the end of each stroke on the
graph (1, 2,3,4,5,6) for these are the points that I will use in my discussion.
The performance of an ICE is expressed in terms of the Thermal Efficiency (TE)
which is the Work produced divided by the total Heat Input (Qin)
to the engine. Theromdnamics can show that TE of an Otto Cycle can be
determined by dividing the heat flow out of the engine (Q out) by the heat flow
into the engine (Q in).

TE = Qout divided by Qin

Note that if Qout = Qin, TE would = 1.0 which is 100% efficient. Since exhaust
gasses are always hotter than the input fuel air mixture, the engine cannot be
100% efficient. The Otto cycle shows us how efficient an ICE with given
temperatures can be. In reality the Otto Cycle has the best TE that a given
engine can operate; however, friction and time delays in heating and cooling
mean that a real engine cannot actually reach the Otto Cycle efficiency!.
However, the Otto Cycle model gives us an idea of how well a specific engines
operation is compared to the best it could possibly be.

The heat flow in (Qin) occurs during the combustion of the gas between points
3 and 4 on the P/V curve. The heat flow out occurs between points 5 and 6 on
the P/V curve. Thermodynamics shows that the Qin is related to the temperature
(T) difference between points 4 (T4) and point 3 (T3) during combustion on the
P/V curve and that Qout is related to the difference between the exhaust
temperature at point 5 (T5) and the intake air temperature at point 6 (T6).
The fuel intake stroke and the exhaust stroke (points 1 and 2 on the P/V diagram)
move gas and are not part of the Otto Cycle. These strokes do consume power from
the engine and do reduce the efficiency.

Therefore Thermodynamics shows that:
Thermal Efficiency %(Otto Cycle) = [1.0 - (T5-T6) divided by (T4-T3)] times 100%
TE = [1-(T5-T6)/(T4-T3)] * 100%

An example: an input air fuel mixture (T6) might be 25 degrees C and heat up
during compression to (T3) 40 degrees C.

The exhaust temperature (T5) might be 80 degrees C

A typical ignition temperature (T4) is about 400 degrees C

Thus TE = 1 - (80-40) divided by (400-80) = 1 - 40/320 = 0.875 times 100%
TE = 87.5 %

Now that you have some background you can place different temperatures into
the Otto model and calculate the efficiency. From these numbers you should be
able to make several different charts showing efficiency versus temperature etc.
I should point out that it is standard practice to use 25 degrees C as the input
temperature so that every one compares engines with the same starting temperature.

Discussion:

The highest efficiency would be obtained with the highest combustion temperature
and lowest exhaust temperature. However at temperatures above 400 degrees C the
fuel air mixture would self explode and ignite without a spark before the cylinder
reaches the top of the compression stroke. This destroys engine efficiency. This
is what happens in high compression engines when they knock. New blends of petrol
(gasoline) have been developed to help prevent pre-ignition (antiknock).
(See the How Stuff works web site)

The Otto cycle model also shows that the higher the compression ratio the higher
the efficiency. However, high compression also leads to pre-ignition and so typical
compression ratios for petrol fueled engines range from 7 to 10. Some new high
performance blends of petrol enable compression ratios up to 12.
(The How Stuff Works web site discusses special high power racing engines)

Removing the lead from gasoline to limit air pollution also requires engines
with lower compression ratios for efficient operation. To overcome this problem
automobile manufactures have reduced the weight and improved aerodynamic efficiency
to maintain overall vehicle performance.
Measuring temperatures T3 and T4 during the compression and combustion strokes is
very difficult. However, engine design engineers have made engines with transparent
cylinder heads made from quartz so that they can photograph combustion with a high
speed camera and measure the temperatures optically with an instrument called a
pyrometer. Perhaps you can contact the Jaguar automotive engineers at Coventry,
which is near Manchester, and obtain some useful information for your project.

Best wishes for you project, and Happy Holidays
Your Mad Scientist
Adrian Popa