Oct 15, 2007

MC Tech: The four-stroke internal combustion engine

What I'm learning this time:
There is a considerable difference in the actual application of the four-stroke cycle, from the simplified one we were taught in school.


You should already know that the Internal Combustion engine, specifically, the four-stroke one, operates on what the Americans like to call the Suck, Squeeze, Bang, Blow format of operation. The process is very well understood by engineers, who like to formalise the four steps by calling them Intake, Compression, Power and Exhaust. The animated gif, by the way, is from wikipedia.

To summarise what goes on inside the engine. The piston moves upwards, reducing the size of the combustion chamber, which is full of air mixed with fuel. This, obviously, compresses the mixture. Then, the spark plug fires, burning the fuel, and causing the air to heat up very fast. The fast rise in the temperature of the air causes the pressure it exerts on the combustion chamber (all sides of it) to rise very fast. The piston, being the only moving 'wall' is pushed down with considerable force. The piston is connected (by an imagintively named bar of metal – the connecting rod) to the crankshaft, which is rotated by the force that the piston experiences thanks to the expanding air. Some part of the rotational energy of the crankshaft goes into driving – though one mechanism or another – the rear wheel(s), the rest of it, the inertial component, pushed the piston back up the cylinder setting up another sequence. The engine designers' job (and the art part of it) is to make the process as efficient as possible with an eye on the design brief – economy, efficiency, performance or any combination thereof. Liquid-cooling, double overhead camshafts, four-valve heads, porting, twin plugs etc are all devices that help the basic process run better.

Now, for a little more detail. The top of the engine, often called the head, has at least two valves. One is to let the air-fuel mixture in (referred to as mixture from here on out). The other is to let the spent gases out. The third (and fourth if you have a twin spark head) hole is for the spark plug, which simply put, ignites the mixture in the first place. For now, we'll not go into how the valves are opened and closed. We'll simply assume that they open and close according to a pre-determined schedule – which we see in a minute.

We will also, for the moment, ignore what all happens as the crankshaft rotates and focus only on the cylinder and its elements. I'm also going to assume this is a simple two-valve four-stroke motor. Also, inside the cylinder, each cycle is counted off in degress of crank rotation. Hence, all events are timed in degrees (of crank rotation.

In the intake stroke, the intake valve opens as the piston moves from the top postion (called Top Dead Center, or TDC) to the bottom position (called Bottom Dead Center or BDC). The motion of the piston creates suction, which draws in fresh mixture through the open valve.

As the piston reaches BDC, the intake valve closes. The motion of the piston upwards, now heading for TDC, compresses the air. The mixture when it is drawn in is roughly at atmospheric pressure. The motion of the piston will compress it by ten or more times in a high-performance engine. Just before the piston reaches TDC, the spark plug fires.

The mixture, now burning, needs some time to complete the combustion. This is usually between 30 and 70 degrees. In this time, the temperature of the mixture goes from more or less atmospheric to thousands of degrees (this is why you've seen those videos of engines on bench tests running glowing red hot). With the spike in temperature comes a rise in pressure – which, really, is the part we're interested in. Good engines see a four or five-time rise in pressure. At full throttle, the original intake mix (at roughly 15 psi, atmospheric pressure), that was compressed, say 10-13 times (150-200 psi) will jump to as much as hundred times the compression ratio (1000-1300 psi).

The piston, meanwhile, passes TDC and the pressure forces it down the cylinder. The piston's motion (and in consequence, the crankshaft's motion) will consume most of the this pressure energy, leaving the spent gases with about 100 psi. When the piston closes in on BDC, the exhaust valve opens to let the spent gases out. Usually, the valves open before the piston has reached BDC to help the gases leave. Once at BDC, the inertia of the crank starts to push the piston back up the cylinder, effectively shooing the spent gases out of the chamber. As the piston reaches TDC, the exhaust closes and the intake opens to start this dance all over again.

Now, as you can imagine (and this is point of this MC Tech post), the process isn't quite this cut and dry. Like the employees of an office, even one's with routine jobs, the process needs time. The mixture, for instance, has inertia like everything else. It needs time to start flowing into the cylinder. And once flowing, it needs time to stop flowing as well. So, it is quite common for the intake valve to actually close after the piston has passed BDC and started the compression stroke.

Further, at the end of the exhaust stroke, it is common for engineers to start opening the intake valves before the exhaust valve has fully closed – a period known simply as overlap. If this were a simple system, you would expect some intake to pass right through and get exhaust-ed. But, the engineers use the exhaust system to correct this.

As the exhaust gases leave the chamber, they are still under pressure and pressure waves emanate from the cylinder. These will bounce off obstructions in the pipe and get reflected. Obstructions include a joint in the pipe (like when two exhaust headers join into one) or the end of the tuned length pipe. When they get reflected, the pressure wave also becomes negative. If the original wave was, say, a suction wave, the reflected one becomes a expansion wave. If the original's a push wave, the reflection becomes a pull. If a pull wave arrives at the right time, it will help the exhaust gases (pull) leave the chamber more efficiently. But again, a pull wave won't sit down having helped the spent gases leave either. It will usually pass into the chamber and then into the intake header, pulling intake gases into the chamber, working, in effect, as a supercharger. Good overlap and exhaust design will create a beautiful torque boost. Good design will also start the lazy mixture to start flowing early, sometimes before the piston has actually arrived at TDC to begin the intake stroke. Early bird gets the burn.

Bad overlap design can happen. A push wave can arrive, carrying the spent gases back into the chamber (push) and reducing the amount of burn-able mixture in the chamber. And like, the pull wave, it will travel into the intake header, slowing or blocking incoming mixture. The result is weak combustion, and a flat spot. You open the throttle in this rev range, and literally, not much happens.

Due the inertia of the gases, this time in the case of the spent gases, the exhaust valve is usually opened early too. Most engines, therefore, open the exhaust valve before the power stroke ends formally at BDC. The pressure energy left in the spent gas helps begin the scavenging process. Whatever gases leave on their own reduce the work the piston has to do while coming back up to TDC in terms to pushing the spent gases out. One could argue that this reduces the efficiency of the power stroke.

However, the exhaust opens just before TDC. At this point, the piston is slowing to a near stop, and the leverage angle (the angle at which the connecting rod is driving the crankshaft) is not really conducive to using the motive force to turn the crank harder anyway. Any force running down the con-rod at this point will only try to push the entire crankshaft away from the piston (down) rather than try to make it rotate. Further, as the piston nears BDC, it doesn't have all of that energy left in any case. So the loss is minimal. The early opening does help with the efficiency of the next cycle, and will usually cause more useable power to be 'manufactured' in the overall scenario.

Oh, and while I never thought this might actually be useful, double-clicking any word should pop-up and explanation from answers.com

6 comments:

The BATFAN said...

I feel like writing the Class X Boards again. Those were the times.

But I hadn't ridden back then :D

Glifford said...

Never before seen footage!. You got to show this!

Link courtesy Sridhar Nivarty!

--xh-- said...

lot of new things for me to learn :) enjoyed it.

first_synn said...

Didn't get time enough ta read it in entirety until now. Thanks a lot for that post. Loved it.

ruffrohit said...

hey there! nice article... hope you cover the SIX stroke engine soon...

Maltesh Ashrit said...

Nice Stuff Rearset.

Can we have something on Fuel Injection Systems, Camshaft Timings, chassis engineering and materials used in engine design and body parts.

Maltesh Ashrit