Engine Durability - Stops'n'Starts

Conventional ICE system weakness in such condition can be due to premature starter motor burn-out.

 

Testing with the earlier 1.5L, NHW11 shows MG1, 18 hp, spins the engine up to ~900 rpm and holds it there for a little under 2 seconds when cold before application of fuel. This brings the oil pressure up for the cold engine before engine power takes over. In ordinary driving, this happens in about 0.250 ms., an amazing feat but MG1 has tremendous power and the engine and vehicle control computers are that good.

So far, we've not heard of any failed 1.5L engines except for an attempt to use diesel fuel (destroys all gasoline engines.) The 1.5L engine, 108 hp at peak power, is running only at 75% power at full throttle so our engine has a very easy life. Remember it is the oil 'film' that does the work and the pump just replaces what seeps out the moving parts. After all, one can run for a while with the oil pressure warning on ... just not for long.

Bob Wilson

 

As Bob said, the reason most engines wear fast at startup is lack of a thick oil film in the bearings while the crankshaft is under load (the burning fuel pressing down the pistons and connecting rods on it). With the Prius, there is no load on the crankshaft until the oil pressure is up, as it is spun by MG1 (a 20 HP electric motor). Even fuel is not injected until the engine is ready for it. Also, a "normal" engine starts at around 500 RPM, which really needs a good oil film as there is more time at low RPM for the forces to press out the oil that -is- there. It doesn't have enough. Prius' engine is running around 2000 RPM when the fuel is injected. As the oil pressure is up and there is a full thickness of oil there will be no abnormal wear.
Further, as the oil pressure is mechanically related to RPM, the "normal" engine starting at 500 RPM (or less) will have much less oil than the Prius starting at 2000 RPM.

Stopping the engine doesn't cause wear issues. Note that the Prius stops the engine crankshaft at a specific position, and this sometimes requires it to "bump" it a few times to get it there. This is so the engine can be started faster.

 

David, what do you mean by "bump"? Could this be what some including me seem to feel as a "shudder". I mainly feel this at startup but have felt it at shutdown too. It has been getting more pronounced as time goes on (and or the air temp going down).

Peter

 

Usually it will pretty much go away once the car is broken in. Pearl did it a lot when new, but I don't notice it much anymore. Note that you will notice more and more "little things" as you get used to the car. So I conclude Pearl doesn't "bump" the engine too much anymore.

Yes, "bump" means it applies a pulse of power to MG1 to turn the engine a bit until it gets to the "magic spot". I forget what "crankshaft degree" it's going for.

And yes, it's a bit worse in colder weather. But it will pretty much go away after a few thousand miles. So don't worry too much about it.

 

The other benefit is the start and stop is that we are dealing with a warm engine. The oil is already warmed up and protecting the engine. The biggest wear is when the engine is cold and all the oil is sitting in the sump. If the Prius engine is pre-lubing / pressurizing the oil system especially with 0W20 oil the wear is very minimal even on a cold start.

I know this since I owned a Volvo turbo. When they did an oil change the mechanic would pull the coil wire and use the starter to pre-pressurize the oil system before reattaching the wire and starting the car.

 

The Prius does not have a Turbo and is not a likely candidate for an aftermarket turbo, this post is just for completeness.

Normal engines may suffer on start up, but Turbos can also suffer on shutdown. If you turn off a turbocharged engine too soon after high load, too much heat may 'cook' the oil and the turbo bearings. It is important to let a turbocharger cool while it still has a steady flow of oil. (If they ever build a Turbo Prius, I hope it has a 201v oil pump to keep oil flowing until the turbo cools; that would also help startups!)

 

Mine isn't. It seems to be somewhere in the 1000-1100 RPM range.

 

Yes it's spun up before the fire is lit, but that doesn't negate wear because there is compression, unless the exhaust valves are held open during starting I doubt that because surely we would have heard about it. I would have expected a pre-oil system, but there isn't one. Maybe it's only cold starts that cause the wear? I can tell you from monitoring what little I can from the bus, the engine does not live a hard life, most of the time, mine is loafing around at low RPM

 

The reason the wear is negligible is that compression forces are -tiny- compared to combustion forces. It's like 100 times or more greater.

When cold a piston engine has "other issues". Such as the pistons are too small and can slap (they expand when they heat up), the oil is too thick and doesn't flow as well as when it's hot (though synthetic oil does minimize this), and many others. Which is why automobile engineers want you to not apply much power until the engine has started to warm up. Also, some of these issues are why the first few shutdowns of the ICE are rough. Once warmed up the ICE shutdown is often almost unnoticeable.

Yes, the ICE in a Prius can loaf along if power isn't needed. I've seen 1200 RPM on the highway (110 km/hr or around 70 MPH). But if power is needed (very steep uphill) I've seen 5000 RPM. So it's not all gravy.

 

There are Prius taxis out there going strong long after a comparable Crown Vic has needed a motor and tranny

What usually kills a Prius taxi is a crash with a stolen car, at least here in Winnipeg, car theft capital of Canada. Ironically, this crash was NOT the result of a stolen car, shocking. This photo is courtesy Sun Media



A home had surveillance cameras and caught the crash

http://www.winnipegsun.com/news/win...tml#/news/winnipeg/2009/05/28/pf-9604316.html