Timing Belt

No Prius has a timing belt, so no. The timing chain may well be good for the life of the engine. I have no idea if it is an interference engine, but it doesn't matter.

 

In case you ever have any reason to remove camshafts, the shop manual instructions contain some mysterious instructions to make sure the crank is ~ 40 degrees or more away from dead center before snugging the bearing caps down and setting the timing, which I haven't been able to make any sense of except on the assumption that interference is indeed possible.

-Chap

 

Where did you find that? It would be valuable to have a reference for such an important piece of information.

Did the source also have an alternate explanation for why the service manuals stress having the crank not be at dead center while adjusting cam timing?

Thanks,
-Chap

 

One should note that there is more than one "Prius Engine" across Gen1, Gen2, Gen3 and Prius C, so YMMV, but since we have a timing chain (not a belt) I am not concerned.

JeffD

 

Toyota engine rule of thumb, if it's belt driven it's noninterference, if it's chain driven it IS interference. Only exception is the 2uz V8 which is a belt driven interference engine, but I doubt anyone here has a Tundra so.

SPH-D710 ? 2

 

Timing belts wear out in most cars at 60k miles (some, like my Subaru Outback, are still good at 100k miles). Timing chains almost never break. If there is a problem with the chain or its tensioner or guides, you will get a warning - lots of noise and you will have time to get the chain replaced.

It's not just the valves that get damaged, the collision when a timing belt fails messes up the pistons as well and your engine is toast.

JeffD

 

Interference means valves will contact pistons if chain breaks. Chains are less prone to failure than belts which is why they are used on interference engines.

SPH-D710 ? 2

 

Chains should be used on interference engines. Several manufacturers use timing belts on interference engines (quieter and less expensive) and owners need to be aware of this issue. Always determine when you buy a car:

1. If the engine is non-interference you can be relaxed about changing out the timing belt as a failure is just an inconvenience (you will need a tow if the belt breaks).
2. If the engine is an "interference engine" and has a timing belt always follow the manufacturer's suggested replacement interval as a failed belt will result in needing a new engine.

JeffD

 

Interference engine: one where the valves (when they open) extend into the same region of space that the pistons occupy at the top of the stroke. In other words, they could interfere with each other if the timing gets off (valves opening at the wrong times relative to pistons moving).

Non-interference engine: has enough space in the combustion chamber for the valves to open and never collide with the pistons even if the timing is wrong.

How can the timing get wrong? (1) by breakage of whatever keeps the camshaft in sync with the crankshaft (belt, chain, or sometimes gears); (2) by the belt, chain, or gears getting loose enough to jump teeth; (3) during any mechanical work when you have the belt, chain, or gears removed and could turn the crank separately from the cams or vice versa.

Obviously (1) and (2) are the accidental failures people worry about. They seem less likely to happen with a chain than with a belt. But if they do happen, and it's an interference engine, damage will result. (And as toyotechwv posted, if Toyota has chosen a chain, it probably is an interference engine.)

(3) is the responsibility of anybody who works on the engine. Any time you have the belt/chain/gears off, if it is an interference engine, it's important not to go rotating the cams except when you know the crank is away from top dead center, and not rotating the crank through TDC for any cylinder unless you know the valves are out of the way (or released by taking camshafts or rockers out). The Prius service manuals do give instructions on how to position the crankshaft when doing certain work involving the chain.

-Chap

 

As an aside, I do not think you can get a 13 to 1 expansion ratio (Compression ratio in an Otto cycle engine) without it being an interference engine. There is no room for the valves to go anywhere.

 

Our Prii engines have a mechanical compression ratio of 13:1 as Jimbo said, but the intake valve closing is delayed in the compression stroke which reduces the effective compression ratio to 8:1 making it an Atkinson Cycle engine. See: http://en.wikipedia.org/wiki/Atkinson_cycle

JeffD

 

As an aside, the Cruz diesel is using a 16:1 ratio. Since the expansion ratio dictates the amount of energy extracted from the hot gas, small wonder we're in diesel thermal dynamic range.

Bob Wilson