Cutting It Close

Cutting it close

TDC

Kevin Cameron

GOOD MORNING, WELCOME TO WORK, now slam down that Starbucks, fire up the CAD/CAM programs and roll up your sleeves. You are familiar with Pro/Engineer software, aren’t you?

Today’s subject is next year’s pistons. As you know, a good compromise on piston design gets harder as engine rpm keeps rising. The reason is that to conduct heat from the hot piston dome radially outward to the cooler cylinder wall, the piston crown and ring belt need to be made on the thick-andheavy side. You’ll recall from Thermodynamics 101 that high heat flow requires a thickness of metal to act as a “pipe” to transmit it.

On the other hand, the inertia loads of constant piston reversals increase directly with piston weight, and as the square of rpm. Therefore, if we keep last year’s chubby pistons and push the redline up from 13,000 to 13,500 rpm, we pay in the form of an 8 percent increase in bearing loads. Naturally, we don’t want to do that, because the Crankshaft guys down the hall, who know that plain-bearing friction increases as the cube of bearing diameter, have just reduced the diameter of their journals by another 4 percent. They aren’t happy to think that the Piston Department plans to cancel their hard-won gains with last year’s unfashionably heavy pistons.

Keeping the crank as durable as before but with downsized journals means getting permission from Accounting to step up the material spec. Melted-down coathangers won’t cut it anymore-now the crank has to be made from one of the HSLA (High Strength, Low Alloy) steels. Good value, but a little more expensive.

So, as a result of raised voices in a few meetings, and perhaps a threatening memo or two across our desks, we agree to lighten up. The pistons, that is. We know from the preliminary performance predictions that piston heat load is going up with the revs on the new models percent more rpm means 4 percent more power impulses per second, each of which drives heat into our pistons.

First we try to keep the same metal in the dome and ring belt, and scratch some metal out of the rest of the slug. This way, the temperature of the top ring will only rise a little bit. If we end up building a synthetic-oil-only engine, we don’t want our department’s name on the decision. But over the past few years we’ve already worked these pistons over thoroughly. Their skirts are so short that when you set one on the conference room table, the wristpin bosses hit first and it tips sideways until a skirt hits. No room for improvement there.

Not so long ago, wristpin bosses grew out of the skirt, but we had to give that up at the last bore increase-if we hadn’t, the wristpin might have weighed more than the piston itself. So now the bosses hang down from the dome like stalactites, completely separate from the skirts, and the pins are little short things. Very light.

We’d like to shorten those stalactites and put the wristpin closer to the piston dome, but then the pin hole would have to go through the ring grooves. We tried to sell Accelerated Durability Test on using just one compression ring, but they outgunned us at the “discussion.” So if we’re going to make lighter pistons, we’re going to have to make the dome and ring belt thinner.

How can we get the heat out? By making all those piston rings and the oil-cooler earn their keep, that’s how. We’re going to install oil cannons in the crankcase and squirt oil up at the undersides of those hot piston crowns. Yes, we have to make both the scavenge and pressure sections of the oil pump bigger to handle the extra flow, but parts are parts. The engine makes the numbers and the piston rings still run cool enough not to stick-even with the cheapest oils that meet our basic spec.

How far can we go with this? Maybe not too far; we’re already running the new generation of low-wall-pressure oil scraper rings, so we have to be careful not to overwhelm them with too much piston-cooling oil. Word got around that Test Division saw more than a full horsepower gain from those rings, so there’s no going back to the old way. We’d like to study internal aerodynamics to see if we can somehow steer the oil away from cylinder walls. Expensive. Everything comes down to money.

What’s in the next wave of planning meetings? Scheduled armwrestling with the worriers from Emissions Compliance. Not that we have a problem right now. It’s meeting the next scheduled tier of regulations. Seems they want to raise the top ring, closer to the piston crown, to reduce the volume of unburned mixture that can get trapped between the ring land and cylinder wall. Big emissions source-the stuff pours out of there after the power stroke-straight out the exhaust and into the EPA sampling tube. Trouble is, the higher we push the ring, the hotter it runs. Then the guys in Warranty start phoning us about stuck rings. How many engines do we want to rebuild for free? Do we now tell management that at last, umpteen-dollar-a-quart synthetic oil has to be mandatory for the new model? (We know it has to happen some day.) No, they’ll never go for it in this decade. Or do we ask the petroleum-oil guys for an even higher detergent loading to keep the hot rings free? We can hear those humorists now: “Do you really want a lubricant, or are you actually looking for more like a soap?” Then the Proctor & Gamble jokes.

Maybe there’s a silver lining. The higher the rings go, the higher we can push the wristpin, which sets free some weight. We can use that weight to conduct heat, or we can save it as a hidden asset for the next round of piston lightening.

Now the scary part: Rumor has it they’re shortening the stroke again in two years. That’ll mean a bigger piston and even higher revs. We’ll think about that tomorrow. Œ