Unseen Drama

Unseen drama

TDC

Kevin Cameron

LIQUID FUEL SPRAYS FROM THE CARBUretor’s needle jet, shooting upward at right angles to the intake air flow, moving at hundreds of feet per second towards the intake valves. The spray is a mixture of streaming fuel and droplets of all sizes. Encountering the intake air, the streams and droplets are physically torn apart by the air’s impact on them.

On the surfaces of all droplets, fuel is evaporating. The bigger droplets, having more momentum, cross the intake flow to splatter against the inside of the intake port, some fragmenting into smaller droplets, some forming a liquid layer on the wall, pushed along by the streaming air towards the valves. The smaller this intake passage is, the higher will be the velocity through it, and the more vigorous will be the process of fuel break-up. This is one reason why many engine designers and tuners are currently choosing smaller, not bigger, intake passages.

Fuel molecules in a droplet are bound together by the cohesive forces between them. To evaporate-to leave the droplet as free, gaseous moleculesthey must overcome these forces. That takes energy. The available energy is heat, so as the fuel evaporates, the air/fuel mixture consumes its own heat-it cools. The layer of liquid fuel streaming along the intake wall picks up engine heat, ultimately raising intake temperature.

Not all the fuel evaporates by the time it reaches the combustion chamber, and the evaporation process is a selective one. Gasoline is not a pure substance, but a mixture of hundreds of molecular species, each with its own volatility. Every fuel droplet whirling down the intake pipe is therefore a tiny distillation apparatus. The easy-to-evaporate fractions flash off early, leaving the dwindling droplets richer in harder-to-evaporate, less volatile fractions. Liquid fuel streaming along the intake wall shreds off into space where the surfaces end at the valve and seat, flinging itself as yet-unevaporated droplets across the cylinder. Some of this evaporates on the way, and the remaining droplets impact on the cylinder wall or piston crown. There they again form a film. Combustion heat will bake and partly burn this fuel to generate a wet “wash” area of shiny black, indicative of imperfect mixture formation.

In the cylinder, the entering highspeed mixture creates a rolling flow, causing the cylinder contents to turn over rapidly. This motion will decay from viscous friction as the piston rises, but what remains of it will create the mixture turbulence that generates rapid, efficient combustion. The piston rises, converting the original rolling motion into a tangled mass of eddies. Of the original high intake velocity of hundreds of feet per second, only 10 percent or less remains. Those fuel droplets still unevaporated are buffeted by turbulence, and they evaporate further.

Ignition occurs just before 30 degrees BTDC, but for a time-perhaps 10 crank degrees or so-there is no detectable pressure rise in the cylinder. This is the so-called delay period, during which the flame nucleus is so small that it cannot grow very quickly. In time, like the early stages of a chain reaction, it reaches a size beyond which its expansion becomes very rapid. As the piston reaches 10 degrees BTDC, mixture trapped in the squish zones-regions where the piston comes very close to the head-is squeezed out at peak velocity. This process picks up a lot of heat from the piston and combustion chamber because of its high velocity, and these hot squish jets are the last chance for mixing the rich and lean zones that now make up the unburned mixture. The heat they gather can hurt efficient operation, for if there is too much squish action, the result can be overheating and/or knock. Some liquid fuel remains on the cylinder walls, scraped up by the piston rings.

Exposure to leftover exhaust gas, and to the hot piston and combustionchamber surfaces, heats the unburned charge. Compression heats it, too, and after ignition, the expanding flame front further compresses and heats it. Temperature is a measure of the velocity of the molecules in a gas, and in the hot mixture, high-speed molecular collisions set the complex fuel molecules into vibration. The weakest structures break, producing highly reactive free radicals. If they become too numerous-either because the mixture is heated too much before it is burned, or because it is held at high temperature too long-parts of the unburned mixture may ignite by themselves. Explosively. Shock waves from this explosive combustion can strike the interior of the chamber, producing the familiar rattle of engine knock.

The flame front, itself a random, whirling storm of fiery tentacles, engulfs the last major remnants of mixture. Peak combustion pressure is reached at about 15 degrees ATDC. Peak combustion temperature may be as high as 2500 degrees K, with peak pressure close to 1000 psi. Some slow combustion continues, consuming what has been hidden from the flame in the recesses of squish, or stuffed into the clearance between piston and cylinder, above the top ring. Any fuel still in a liquid state on the cylinder wall continues to evaporate, but finding no oxygen with which to combine, it is simply pyrolyzed-broken down by heat into some of its constituent parts, including free carbon. These are the infamous unburned hydrocarbons. Some dissolve in the oil on the wall, and are scraped down to the crankcase to blacken the lube oil. Other fuel has failed to react because the mixture zone in which it found itself during combustion was either too rich or too lean for complete combustion. This is the result of imperfect charge mixing; the proportions of fuel and air can be correct on the average, yet still be rich or lean locally.

The exhaust valve opens. The players leave the stage. □