There are far more details to engine design than can be discussed here, however here are the basics for 2 stroke
engines, many of the principles apply to 4 strokes etc.
MODERN INDUCTION SYSTEMS
Almost all ‘sports’ engines feature ‘front’ induction, where the fuel / air mixture enters through the hollow
crankshaft. Most racing engines feature ‘rear’ induction, of several types. The more common is the disk
induction where an extended crankpin drives a ported disk which opens and closes the inlet.
The less common rear induction is the ‘drum’ system, which in turn has 2 types, the ‘normal’ and ‘reverse
drum’. Instead of the crankpin driving a disk it drives a smaller version of the hollow crankshaft.
The conventional drum has the fuel / air mixture enter via the drum centre and exit vertically in a manner similar to
a front induction set-up.
The reverse drum has the fuel inlet from above (or below) in a manner similar to a front induction set-up and
exits through the centre of the drum. This system has the advantage of providing fresh fuel (oil) to the crankshaft /
conrod which is why it is very popular for C/L team race engines.
LINER PORTING TYPES
Almost universal nowadays is the Schnuerle port liner as opposed to the old loop scavenged , cross flow or
piston port systems.
The loop scavenged system used one large transfer port and one exhaust port on the opposite side. Some times
these ports were split into several sections. The intake charge was prevented from exiting straight out the exhaust
by using a piston baffle.
The cross flow system which was employed on many diesel engines where a transfer port was used front and
rear combined with an exhaust used on either side of the liner.
Piston port induction as per the famous Mills engines is where the fuel induction is directly into the liner.
Several manufacturers have recently introduced a reed valve induction system, via the engine back-plate. Not
totally new as it was done by Frog many years ago (although in a different fashion).
Schnuerle porting uses a group of transfer passages arranged in the liner as to direct the incoming mixture away
from the exhaust port (infact often crashing into each other). Over the years the system has seen many
configurations of transfer port numbers etc, but the same principles apply. The classic Schnuerle port liner has 1
@ exhaust, 1 @ boost (opposite to the exhaust) and 2 @ transfer ports on either side.
PISTON & LINER MATERIALS
Most early engines and a decreasing percentage of modern engines featured a cast iron piston in a steel bore.
Many of these engines feature a range of different ring types to provide the seal.
A more recent and now more common engine is the ABC, ABN and AAC piston and liner. Some of these
engines are ringed.
The ABC engine has an aluminium piston operating in a hard chrome bore, brass liner.
The ABN engine has an aluminium piston operating in a special nickel plated bore, brass liner.
The AAC engine has an aluminium piston operating in a hard chrome plated bore, aluminium liner.
METALLURGY
The reason for all of the above liner configurations, apart from using materials compatible for wear and long use,
is to provide for the different expansion rates of the components due to heat (combustion).
If an engine used the same material for the piston and liner it would seize almost instantly as the piston is subject
to higher heat, hence expands at a greater rate than the liner.
In the case of the AAC engines, the piston is made of an aluminium featuring a high percentage of hard wearing
silicon in the alloy. This alloy expands at a lower rate than the lower silicon alloyed liner.
In theory the ABC, ABN, and AAC engines should never seize as the liner will expand away from the piston as
heat rises.
The major difference between good and bad engines is the metallurgy and quality of the component fits.
TOLERANCES & FITS
Whilst it is not easy to see without a trained eye, the quality of component tolerances, shape and fits is the most
important area of all engines.
It goes without saying, but often not the case, all the major components of an engine should be round, i.e. the
liner fit in the case, the bore, the piston, the head and the bearing housings. A close examination of a liner will
often show an uneven wear pattern, i.e. the piston and / or liner are not perfectly round or operating in a very
uneven cooling system.
The crankshaft often rubs on the case between the bearings due to the flexing loads imposed on the crankshaft.
All engines should have the case diameter of this area enlarged (most manufacturers do this).
The liner of an engine should not be a perfect cylinder. The cylinder should taper to a larger diameter at the
bottom (only the top of the liner is critical to the compression seal and power stroke). The average taper is
between .002 to .003 of an inch per inch of liner height.
The piston should also be tapered, although only the ‘better’ engines are tapered, or barrelled as it is often
referred to, because the piston should in a lesser way be shaped like a wine barrel, i.e. a lesser diameter at the
very top and bottom of the piston.
The reason for the piston to be barrelled is 4 fold.
1.To reduce contact friction with the liner.
1.It is best to have a small gap at the top of the piston to form a sealing
oil wedge.
1.The top of the piston is the hottest, causing the piston crown to
expand.
1.To help prevent a piston edge catching in a liner port. As the piston
travels up and down there are many side loads etc which can force
the piston sideways slightly. This can be a problem for racing engines
which generally use very large exhaust ports.
Crankcase vs. crankshaft
One of the biggest problems for engine designers is the expansion coefficient difference between the aluminium
crankcase and steel crankshaft. As engines are ‘fitted’ cold, under operating temperatures the case will expand
(lengthways) more than the crankshaft. This puts an uneven load on the bearings.
Many competition C/L team race engines have gone to great lengths to improve this problem, including the use
of all steel front section of the crankcase. Some of the new Webra engines use a special low expansion
aluminium crankcase, and it shows in excellent performance.
It is important that the ‘front end’ of an engine receive cooling air.
TIMINGS
The duration of the opening of the intake, transfer and exhaust ports (timings) is dependent on the end use of the
engine. Racing engines often have very high timings (generally quoted as the degrees of rotation for which the
port is open). The shape of the ports can also be very important.
As a general rule, the manufacturers do a good job of providing well designed ports, normally the only time the
end user can justify modifying a port’s duration is to match an engine to a tuned pipe. A good example of this is
some of the larger 2 strokes above .90 size, where the manufacturer has used low timings to increase torque.
For an engine to respond well to a pipe, the exhaust timing must be no lower than 145 degrees. Below 140
degrees, the pipe can actually deteriorate the engine’s performance.
Hot Links and
Recent updates / Added MPEG files
| RC Items for Sale | Whats New in RC | Latest RC Books | Tech Tips | RC FAQ | RC Greetings | Top RC Sites |
| Top RC Banners | Other Links | RC WallPaper | RC News | Free RC Software | RC Videos | Things to Know |
| RC Reviews | RC Articles-Car | RC Car Setup Sheets | Kyosho Setup Sheets | RC Manuals |
| Top RC Pick | Latest Sports | KX-ONE / V-ONE-S Snaps | V-One-S Snaps | Track Snaps for Aug 02 |
| Track Snaps for Sep 02 | Track Snaps for Nov 02 Part I | Track Snaps for Nov Part II |
| Nad Al Sheba flying Field Snaps for Oct 02 | Exclusive Mustang Snaps for Nov 02 | Exclusive MPEG Movies |
| Nad Al Sheba flying Field Snaps for June 03 | Nad Al Sheba flying Field Snaps for August 03 |
Copyright ©2000-2003 Dubairchobbies.org. All Rights Reserved. A website of Dubai RC Community.
1024 X 768 / configured for I.E 5+ for best view
Read the Disclaimer
