How Internal Combustion Engines Make Power

To operate an internal combustion engine needs fuel, air (together these make up mixture and must be in a narrow range of ratios), and ignition in a narrow band of timing. These specific topics are taken up elsewhere on this site, so we'll concentrate here on other particulars of the process: how various parameters affect how much power is made and where on the powerband.

First: displacement gives power accros the range. The more swept volume in the cylinder(s) the greater the power. The same with (carefully controlled) compression. The smaller the volume at Top Dead Center, the more power accross the board. Larger swept volume plus smaller combustion chamber the more power - at the bottom; and on the top. The cost? High compression is hard to start. It's sensative to fuel quality. It requires a lot of spark advance; enough compression and the advance is so great that the engine doesn't know which way to turn.

This can be mitigated by opening the intake valve(s) earlier and closing the exhaust later effectively lowering the cranking pressure. However, long cam timing costs low-end power and at high RPM it has the opposite effect on cranking pressure. Overall, we seek to increase the peak power and to raise the the achievable RPM.

Generally, low-end power is enhanced by skinny ports and small valves and very little valve overlap. Higher peak power and a willingness to rev are governed by the opposite. A low RPM torque peak can be acheived by high compression, a skinny exhaust and small carburators or throttle bodies. The higher the peak power - generally - means that there is only a narrow band of power: note the current generation of 700+ horsepower cars - slugs out of the hole; rockets at redline.

To confuse matters, simply rotating the cam(s) forward (in the direction of normal rotation) moves the peak power up the rev range; advancing it/them (the opposite of the above) brings the peak lower. Here is where you can start leaning about cams

More to come . . .

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