An engine’s power can be increased
by increasing the amount of air flowing
through it and/or the brake mean
effective pressure. The amount of air
flowing through a piston engine is
determined by the size of the cylinder
(displacement), the air density, and the
rpm. The brake mean effective pressure
is determined by many factors including
fuel heating value, air density, and the
efficiency of all the various processes.
These factors were recognized very
early, and most racing classes limit an
engines power by limiting the displace-
ment, air density (amount of super-
charging), and sometimes the fuel. That
leaves increasing rpm as the only way to
move more air through the engine.
Increasing brake mean effective pressure
is a lot harder, especially when fuel type
is limited.
As was mentioned in part one, a two
stroke’s power is limited both by
breathing (the ability to handle more air)
and scavenging (the amount and purity
of the mixture left after the exhaust port
closes). Brake mean effective pressure is
closely related to scavenging efficiency
when everything else is equal. Even with
major improvements over the last 50
years, loop type scavenging systems
can’t compare to a four stroke’s
scavenging efficiency. Our model
engines along with small industrial two
strokes have been stuck at break mean
effective pressures of under 100 psi (7
bar) for a long time. Naturally aspirated
four strokes seldom exceed 150 psi (10
bar). The best racing two strokes can
reach 200 psi (14 bar) but the pipe is
supercharging the cylinder. A top fuel
dragster engine can reach 1500 psi (100
bar) with a combination of high super-
charge and nitro.
Let’s look at some other two stroke
scavenging systems. Uniflow
scavenging, where the incoming air
enters at one end of the cylinder and
pushes the exhaust out the other end, is
the gold standard of two stroke
scavenging systems. In the past, three
methods of uniflow scavenging have
been used. The poppet exhaust valve
engine, the double piston engine, and the
sleeve valve engine. See the pictures on
the following page.
Continued on page 4
PROPWASH
April 2013
3
policy to pay first. Then, once your primary policy has paid what it covers, NAMBA’s
secondary policy goes into effect. This would cover such things as deductibles,
amounts over the policy limits, etc. If you do not have health insurance of any kind,
NAMBA’s policy covers you the same as if it were a primary policy. There are of
course deductibles on both of these coverages. $500 on the liability coverage, and
$100 on the personal accident coverage. However, in the past, NAMBA has been able
to pick up the cost of these deductibles, and we assume we will continue to do so in
the future as long as funds permit.
As you can see, your NAMBA insurance provides you with the best protection
available through any of the model boating organizations. Hopefully, you will never
have to make use of this coverage, but if you do, you can be assured that you are
covered. Continued availability of this exceptional coverage is of course to some
degree up to you. Make sure that you are aware of all of the safety regulations, and use
common sense in your running. Preventative medicine is always better than having to
resort to a cure. By avoiding needless accidents, we can insure that the coverage will
be available when really needed.
High Power Two Stroke Design Part
2
By Lohring Miller
NAMBA Safety Chairman
The following is part two in a series of three articles. Please go to the November 2012
Propwash on the NAMBA web site if you missed out on part one.
Before we continue our look at future directions in two stroke design, we need
some basic definitions and some engine history. I’ll use British engineering units for
these examples. Engines were created to do work. Work is defined as moving a weight
a distance, measured in foot (the distance) pounds (the weight). Power is how fast this
work is done or foot-pounds per second. One horsepower is 550 foot-pounds per
second. Since we’re racing, power is the basic measure of engine merit we want to
look at. The torque in foot pounds or amount of work an engine can do is found at a
particular rpm by dividing the horsepower by that rpm and multiplying the result by
5252. This torque can always be increased with gearing to match your needs, but the
power will not increase. Therefore, the work won’t get done any faster.
Internal combustion engine power is closely related to the amount of working fluid
(usually air mixed with a little bit of fuel and/or combustion products) passing through
it. In piston engines this is the piston’s displacement times the rpm. The combustion
process in the cylinder increases the pressure of this air and it expands, pressing the
piston down. The result of one revolution is a cylinder that decreases then increases in
volume with a varying internal pressure. That’s shown on the pressure volume
diagram below. The area inside the graph represents the work produced in a single
cycle. The average pressure acting on the piston is called the mean effective pressure.
A more easily calculated figure of merit is the brake mean effective pressure, or the
average pressure acting on the piston needed to produce the power measured on a
dynamometer.