The way an Engine Works.

Explanation of a Piston-Type Engine

Our piston type engine transforms the kinetic energy of exploded fuel into mechanical energy by confining the expanding gases in a cylinder so that they will thrust a piston (fitted into one end of this cylinder) outward. Each explosion of fuel results in a straight-line outward movement of the piston which we call a power stroke.

This power structure is a single, forceful thrust that is limited in duration and also limited to the distance that it is practical to allow the piston to travel in it straight-line movement. In order to develop mechanical power (the capacity for continuous work) it is necessary for our engine to accomplish two things:

1.It must transform the straight-line piston movement into a movement which can be harnessed to various types of work.

2. It must provide a series of power strokes that will continue as long as desired.

The basic design of a piston-type engine

We harness the straight-line piston movement by transforming it into a rotary movement (of a shaft) that becomes continuous in one direction of rotation. The piston is connected to one end of a simple lever that has a fixed fulcrum at the other end. During its power stroke the piston revolves this lever through a half circle (180 degrees), at which point the lever stops the piston movement. With the power cloak thus ended, the rotary movement of the lever carries it on round the remaining 180 degrees of a full circle-and the leader returns to the level position which also returns the piston to its starting point.

In a piston-type engine, the cylinder is a cylindrical bore inside a cylinder block. The piston is a metal cylinder fitted into this cavity and encircled by piston rings which seal it tightly (so that the expanding gases cannot escape around it) while also serving as bearings for the piston to slide on when it moves in the cylinder. One end of the cylinder is sealed closed by a cylinder head which contains a mated cavity called the combustion chamber (in which fuel is exploded). This cylinder head may be a separate parts bolted to the cylinder block with a cylinder-head gasket in between (to seal it tightly); or it may be an integral part of the cylinder block. The opposite end of the cylinder opens into an area enclosed by a plan, called the crank-case, that is fastened to this end of the cylinder block.

Instead of a simple lever with a fixed fulcrum point we use a crankshaft held in a stationary support by one or more main bearings which allow it to revolve. The "lever" is an offset (throw) of the crank shaft (which "juts out" at a 90 degree angle to the shaft center line). Our piston is connected to the shaft (crankpin) of this offset by a connecting rod that is held to the crankpin by a connecting rod bearing. A wrist pin with bearings serves to attach the piston to this rod.

For each single power stroke to occur there must, of course, be a refresh supply of fuel (a charge) in the combustion chamber when. This charge must be compressed, to obtain its maximum power. When it is ready, combustion must take place-followed by the power stroke. Finally, the used charge must be expelled to make room for the next fresh charge.

There are five events that must occur in any piston type internal-combustion engine; they constitute what we call a cycle. They are:

1.Intake-the filling of the cylinder to capacity with a fresh charge of fuel.

2. Compression-the compression of this charge into as small a space as practicable.

3. Ignition - the firing of the compressed charge.

4. Power stroke-the resulting outward thrust of the distance.

5. Exhaust-the discharge of the burnt gases from the cylinder.

The vacuum principle of intake.

If a piston were designed to completely fill its cylinder when in it, and it was pulled partially out of the cylinder without any air meeting passed it - the space that it had occupied would be completely empty ... what we call a 100 percent sign vacuum. Actually, the piston of an engine does not fill its cylinder (It never goes into the compression chamber); and there is always a little leakage around it, even when the piston rings are in good condition. Therefore, when a piston is moved outward, only a partial vacuum is created.

This partial vacuum, however, contains many fewer molecules of gas, ... than would normally occupy the space. In short, there is room for many more molecules. If an opening is provided to connect this relatively empty space with the upside atmosphere (which, we must remember, is under 15 lbs. per square inch pressure at sea level), as much air as the 15 pounds . per square inch pressure can force in will flow into this space. This is what we call the suction of a vacuum. The amounts of air (number of molecules) that will be "sucked" in per second depends upon the outside pressure (which, as already noted, will be less at higher altitude) and upon the density of the outside air (which is also less at higher altitude).

In an engine, intake is effected in this manner. The piston is called outward by revolution of the contrast to create a vacuum instead of pure air, however, a mixture of air and petrol (under atmospheric pressure) is forced in through an opening into the "emptied" cylinder. At sea level more mixture will be forced in - in a given time then at higher attitudes; less then at lower altitude.

An explanation of compression

To repeat what was previously said, in a slightly different manner, compression increases the force of combustion.

It increases it, first, because compressing the charge very quickly super-heats it throughout so that every molecule of the charge is approaching its flash point at the instant combustion starts. Hence, when combustion does occur it is practically instantaneous and compete for all of the charge.

It increases it, second, because the tightly packed, highly activated molecules (all "striving" to move apart with considerable pressure, even before combustion occurs) served as millions of tiny "spring - boards" to thrust the piston back upward.

An engine will run on on compressing charges. The earliest engines were so operated. However, the power output was exceedingly low and wasteful of fuel. we have found that even though some energy is used to "build-up" compression, the power advantage gained far exceeds the power spent. Modern engines are being designed for higher and higher compression ratios.

An Explanation of Ignition.

As already mentioned, our piston-type engines to not use compression as the means of firing the charge. They do not, chiefly, because the very high degree of compression required to create combustion would make it necessary to build over bulky and heavy engine parts to withstand the strain of the high pressure involved. There are other reasons, also, concerned with the types of fuel, its handling, performance-factors, etc.

Apparent in gasoline engines we use an electrical spark, created across the electrodes of a spark plug, to ignite the charge made ready by compression. Electrical energy (a charge) is made to jump the gap between the two electrodes. In jumping the gap it lessens the molecules of the charge that are in its path with sufficient speed to burn them. The "fire" then spreads throughout the charge.

Actually, then, combustion starts at a single point. This point (the location of the spark plug) is carefully plotted for maximum efficiency-that is:
1) so that the combustion will spread accordingly to a plan that will ensure continued, even pressure all over the piston top throughout the power stroke; and
2) so that the fuel charge will be indicted and expanded during the incredibly short interval of time allowed.

The principles involved in exhaust

The "law of inertia".

We have found that any body that is at rest tends to remain at rest, while any body that is in motion tends to continue moving along the same straight line ... and it requires an equal and opposing force to change either condition. This is called the law of inertia. It means simply that if you start an object moving (assuming there is no friction or gravity to interfere), it will take the same force to stop it that was used to start it.

This rule applies to molecules as well as to whole objects. Consequently, once a gas (for instance) is started moving in a direction, each and every molecule of the gas will tend to continue moving in this direction-until stopped by other forces (such as gravity or collisions with other molecules).

Inertia hopes produce a "better" exhaust.

The inertia principle is used to discharge burnt gases from an engine cylinder. As we have said, an engine piston does not fill its cylinder, even when all the way in. If it did fill the cylinder, it would be easy to squeeze out all the burnt gases. However, it never enters the combustion chamber; yet it is desirable to completely empty the cylinder and combustion chamber of burnt gases, to make room for a new charge.

When the crankshaft returns a piston to its starting point, the fast moving piston starts the burnt gases in the cylinder moving at high velocity. Each molecule of the burnt gases is given a very forceful "push" straight towards the combustion chamber. We provide an opening in the cylinder head-and the "resting" molecules flows out of this opening. If the opening is properly positioned, nearly all of the molecules in both the cylinder and the combustion chamber will flow out-even though the piston does not actually "squeeze" them all out.

The "valves" of a 2-stroke-cycle engines.

Valve types

A 2-stroke-cycle engine requires two valves per cylinder-the same as a 4-stroke cycle engine - plus one additional valve to admit fresh charges into the crank case. Hence, there are three valves for a one cylinder engine, and 5 for two cylinders.

The first two are always opening through the cylinder which are covered and uncovered by the piston. These are called the transfer and exhaust ports. For the third valve, a third port, a reed-valve, a rotary valve, or a poppet valve may be used. Hence, we generally speak of a 2-stroke-cycle engine as being a 3-port, read-valve, rotary valve, or poppet valve type. (Note - twin cylinders are an exceptions to this rule).

The transfer and exhaust port

These ports on are generally located at opposite sides of the cylinder - and the exhaust port opens to the "outside", while the transfer port opens into a passage way to the crank case enclosure. Piston length is such that the piston will still fully cover both parts when at T D C - but will fully uncover both port when at the BDC. The exhaust port is usually just a little higher in the cylinder then the transfer port, so that on a down stroke-exhaust begins a fraction ahead of cylinder entered. This also results - on an up stroke - in closing the exhaust port a fraction later than closing of the transfer port . . . which permits the fresh charge in the cylinder to "push out" practically all the remaining exhaust gases.

Either one or more openings may be provided to serve the purpose of each port.

Reed Valves

A reed valve is simply a flap, fastened along one edge, that will flop open under pressure to expose an opening. The flap is called a reed. It is mounted on a reed plate provided with a suitable opening. This plate is fastened to the crank case (at any convenient position) with the reed inside - so that when pressure inside is highest the reed is pressed against the plate to close the opening; but when pressure inside becomes less than atmospheric pressure the valves open.

One, two or more reeds of an "springy" material may be used; and the holes in the reed plate are smoothly surfaced so that the reed (s) will close them tightly. On high-speed engines - likely to develop constable suction inside and therefore considerable outside pressure to open the reeds - each read is provided with a stiff metal stop that limits its opening (to prevent damage to the reed and malfunction). Stops are usually omitted from slow-speed engines.

Rotary Valves

A rotary valve is generally of the integral Type - meaning that it is attached directly to one end of the crankshaft - though some are run on a separate shaft geared to the crankshaft. The valve proper is called the rotary valve - and it rotate against a wear plate which, in turn, is secured stationary to the crankcase side. When the valve and plate holes are aligned, the valve is "open"; and when they are not, the valve makes a tight seal against the wear plate.

The valve may be secured to the crankshaft in any manner. One manner is to employ a key way so that the valve will turn with the shaft, yet be free to move slightly forward or back on the shaft. A spring washer on the shaft between the valve and a shoulder of the shaft serves to continually thrust the valve firmly against the wear plate.

Timing and Maintenance

There are no timing problems with ports or a the reed valve; and the only timing required for a rotary valve is to position it correctly on the shaft.

A reed valve does have replaceable parts - and these are such that it is better to replace damaged ones then to attempt repairs. The openings through the plate and reed must be kept clean, and the contact surface of the plate must be smooth to effect a tight seal .A reed must have the flexibility intended, or it will not function if bend or damaged. If reed stops are used they must not be bent from their original shape.

All parts of a rotary valve are also replaceable - and are generally best replaced rather then repaired. The openings must be kept clean, smooth and a tight seal is preferable. If there is a spring device to hold the contact parts they must be readily changed or checked.

Ignition Timing
Combustion lag

If we were to compress the air-fuel mixture to its flash point, it is not true that the electrical ignition could be termed an "explosion". Though ignited internal combustion may appear to occur in one single flash, the burning actually starts at the electrical spark - and "spreads out" through the balance of the mixture.
For this reason, there is a slight lag between the instant in which the spark occurs and the instant when an explosion reaches it peak force (the peak of combustion).

Combustion lag and Engine speed

This combustion lag affects an engine in several ways which must be compensated for - and the compensation is called advancing the spark (that is, dining the spark to occur ahead of TDC, while the piston is still completing it's compression stroke). When the spark is "moved back" to occur at all behind TDC, we therefore call all this a retarded spark.

A retarded spark is desirable when starting an engine or when it is idling. During starting, if the peak of combustion (or even the initial force of combustion) were to occur before TDC, the comparatively small force applied to start the engine would be overcome - and the piston would be driven back down before it could reach TDC . . . in what we call a kickback.
At idling speed, there is insufficient momentum to carry a piston through to TDC against the full peak of combustion - and an "idling" piston is still moving slowly enough for the combustion lag to be negligible in comparison.

As an engine is revved (speeded), however, the speed with which a piston travels soon reaches a point at which the piston will move a noticeable distance during the split-second interval of combustion lead lag. At 5000 revs per minute, a piston can travel to 10% of its stroke during the interval of combustion lag. then, too, at such a speed the pistons momentum is quite enough to overcome the relatively slight force exerted by the gases burnt prior to the peak of combustion. therefore, it is desirable to advance the spark to that point at which the peak of combustion will occur when it will do the most good - that is, just after TDC.

Back to Natal Karting Home Page