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Monday, March 01, 2004

Feeling tired last night, I decided to commit to doing a minor bike repair (spoke replacement) at 0500 this morning. Good choice: I went to sleep about 2200 or so, got up early, and took care of it.

There's a certain serenity to working that early. Even though The Lovely One and I were both up (her normal work shift is 2400-0600), it felt quiet. Maybe the world outside the house moves more than I think, and makes more noise than I suspect. The noisiest things outside this morning were birds.

The repair went well. Rode the bike to work. Now I'm mulling over the question of whether the Tercel is suffering from a fuel-injection relay or a broken timing belt. Sure hope it's a non-interfering motor.

Hm. I was going to just link to a good definition of a non-interfering motor, but Google doesn't seem to have one. So here it is:

A normal overhead-valve Otto-cycle 4-stroke internal-combustion engine (follow the link for a detailed explanation of how these beasts work; this is the type of engine found in the vast majority of modern cars, excepting only oddities such as the rotary-powered Mazda RX-8) pushes valves down into the combustion chamber during the intake and exhaust cycles. On some engines ("interfering" or "interference" designs) the valves go in far enough or the piston comes up high enough that they enter the space that the piston would occupy at top dead centre. This doesn't matter under normal operation, because when the valves are open, the piston is near the bottom of the cylinder.

The problem arises when the timing belt (or chain, or more rarely, gears or shaft), which keeps the valves synchronized with the pistons, breaks.

This is always bad: your engine pretty much instantly stops running, and you stop going anywhere very quickly. Repair involves replacing the timing belt, which ranges from being a fairly easy fix to a pretty big pain.

On an interference engine, this is very, very bad. The problem is that in most multi-cylinder engines, there is virtually always at least one cylinder which has a set of valves open for any given position of the camshafts and crankshaft. Because the pistons do not stop as quickly as the camshafts (they're no longer in sync, and the pistons get driven by the wheels until you put the car in neutral), they get in a few hundred or thousand rotations while you slow to a stop. And the first time the piston comes up to top dead centre in a cylinder where the valves are down, the piston hits the valves. This makes a bad noise, badly damages the valves and often damages the piston. Fixing these things entails major engine work and costs lots of money.

So, why build an interference engine? Performance. There are good performance reasons (read: high compression ratios) to make the open area above the piston at top dead centre as small as possible. And the valves have to push fairly far into the combustion chamber to allow good intake and exhaust flow. So what this means is that high-end engines are often interference engines (it also means my pedestrian 12-valve 1500cc Tercel almost certainly is a non-interfering engine).

The moral of the story: the service intervals for timing belts are the highest-priority maintenance item for most cars.

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