Prius Prime cylinder wash-down potential issue

I watched the video when it came out last week and considered the same things you are. I came to a couple conclusions that I think are pretty safe.

1. There's one big difference between the Avalon in the video and our Priuses: It's not a hybrid and ours are. That Avalon was never designed for the extreme short cycling that likely led to the problem. AMD theorizes that it was started and only moved to a new parking location and then immediately shut off several times. Either by the shop while working on the first A/C issue, or by the owners that had multiple vehicles and needed to shift them around from time to time. And then the car sat for weeks or months with all that fuel slowly diluting and washing the oil off the cylinder walls. That's a pretty unique situation the car wasn't designed for.

Plus, the way a hybrid starts up with the ICE almost instantly at idle RPM means less chance of fouling the plugs with loads of fuel. In the case of the Avalon, each time it was cold started, the injectors sprayed loads of fuel into the cylinders while the starter cranked at a few hundred RPM. Because the car was shut off so quickly after that, some of that excess fuel didn't have a chance to burn off. That doesn't happen as much in our hybrids because they use the HV battery and the main motor to spin up the engine for an almost instant start.

So in our case, Toyota knows hybrid engines are going to deal with short cycles on a daily basis and designs their engine management systems with that in mind. One additional element of that is that I believe the ECU forces the engine to run a full warmup cycle the first time it kicks on during a drive. That cycle is more than long enough to burn off any excess fuel and fully coat the cylinders with fresh oil.

2. The importance of weekly HV drives and 6m/5k mile(8k km) oil changes. Even with the engines in hybrids being better designed for short cycles, it doesn't completely solve the issue and we still have to deal with increased fuel and water contamination of our oil. A weekly drive in HV mode for PHEV owners helps keep the engine lubed properly, and getting the ICE up to full operating temp can cook any accumulated condensation out of the oil. It won't take care of the fuel in the oil, but that's what the oil changes are for. Personally, I feel that any Prius owners(and especially Prime owners) who push their OCI out to 1y/10k mi(16k km) just because the ICE isn't used as much are being penny wise and pound foolish, but it's their car and their money, and it's totally up to them.

 

It feels so short in the Prius Prime sometimes. I sometimes run the gas engine before I get on the highway so that it's warmed up ahead of time before the first onramp, and it's already cycling back to EV after a couple of minutes. I imagine the ECU is carefully tuned to optimize this forced warmup time based on operating conditions, but sometimes I am shocked how few minutes the engine needs to run, even in HV mode on a Prime.

 

Personally, my gen5 Prime, even in HV mode won't reach full operating temps before it starts cycling the gas engine on and off. The only way i found that keeps it alive enough to fully get it up to optimal temperature is by initiating the Charge mode. Only by doing that will the engine runs continuously, if not, it will drain about 2 or 3% battery, start the engine to replenish those 2 or 3%, stop the engine...so on and so on...

 

Define "full operating temps".

What makes you think you have a better idea of what "optimal temperature" is than the car?

If it's happy to turn the engine off, it's happy with the temperature. If there's any definition of "full operating temps" in PriusChat, it would be when the hybrid system reaches "stage 4" and it starts freely turning the engine off and on.

I suspect that what you're doing is assuming that the steady-state thermal equilibrium temperature you see in a continuously-running engine is "optimum", rather than just what natural arises from the thermal management in such a system.

In a continuously-running system, they'll have made a judgement call that it's not worth spending extra cooling effort to keep it cooler while running.

In the hybrid system, they'll also have made a judgement call that it's not worth spending extra engine-running effort to keep it warmer.

If there is an "optimum" it's likely to be somewhere between the two, and the hybrid could well be closer.

 

In another thread thread, their concern was over the engine remaining hot enough to boil off the water that gathers in the oil.

 

I'm pretty sure the Toyota engineers got it right on the required engine temperature and run times. I know this to be true because the CAT needs a minimum temperature to function properly that means that it wouldn't pass strict emission standards either here or Japan, if the engine is too cold. IMHO, that should be enough to boil-off fuel and water. Longer runs to get rid of moisture.

 

It isn't for the Primes.

5k Gen 3 Prime — Milky oil | PriusChat

 

My plan is to add a 1,500W charger to my (ok, I have an Avalon hybrid, but kind of the same idea) car. That way I can heat the engine up via electricity up to around 180 °F before even starting it.

Right now, with the "stock" block heater, I'm still getting to work with around 100 °F engine temps. Not good!

 

I've installed an oil pan heater on my 21 Prime to boost oil temps when cold! Very effective!

 

I think the problem with the prime, is people perhaps not anticipating the distance to zero EV range. Especially in the winter, when its bitter cold, the engine clicking on for maybe, 3 to 5kms of mixed low speed driving with engine on/off, it likely can't hit temp. Because you're shutting the car down before the engine on warm up cycle can complete *and* run sufficiently to boil off water vapour. If it hits temp when you're 2 minutes from home, and you park then shut it down - the car can't do its job.

My experience getting the car in winter, and doing most of my driving on EV, I made sure to run a full commute of 20-30 minutes once a week with HV mode. Even if it was 60-70% EV ratio due to traffic, it was enough to avoid milky oil. I was careful to make sure I had more than enough EV range to get home. When the drive was healthily done round trip in the EV range no problems. But if I was going to be on the edge, I didn't want to regularly only have the engine on for a super short time and then shut it down at destination when it was regularly -20C or colder over and over again. Once in a while, probably fine. But every day, that's gonna be the milky oil in winter IMO.

 

Any PHEV can have can the engine run for what would be a short trip for a car without a plug. Though it can happen for reasons beyond not having quite enough EV range to get home. Weather and climate play a part. Humid regions will see water get into the oil over time. With the air intake opening facing forward, perhaps driving around in just EV can lead to more moisture getting pushed into the engine than if the car was parked. The throttle plate doesn't form a hermetic seal when the engine is off.

Running in HV mode once in a while to boil off the water will work. If the trip itself isn't a short one. Charge mode might do a better job in that case. It will run the entire trip, and at a higher load, which will generate more heat.

 

In bitter cold weather I have the propensity to run the car in HV mode, just to warm up the cabin. The seat heaters and heat pump just doesn't cut it. Since your layered-up IMHO the seat heaters take too long to reach my body. A side benefit is the cabin warm air will help heat-up your traction pack.

 

You're correct with the Gen4 but the Gen5 battery pack is outside under the car. Like a true EV and is heated with resistuve heaters and coolec with the A/C. Both my Primes started the motor the second i pressed the defrost button so i had warm air anyway. But to counter the crap in the oil, Charge mode once a week it's going to be!

 

Cats aren't all that long to heat up to prerequisite temps on modern engines with variable cam timing. Let the exhaust cam open early, throw burning fuel in those cats and Boom, you get a hot enough cat in no time. The best platform to hear this phenomenon in action are the Mazda Skyactiv engines. When you start them cold, they do quite a racket and the second the cams get normal timing they get surprisingly quiet. A friend of mine who is a service manager at a Mazda dealership told me that when they quiet down, those cats are active and the efi is in closed loop.. takes.. 20.. 30 seconds, maybe a bit more in cold temps but not that long more. Engine oil, comparatively, takes about 15 to 20 minutes of continuous run time to fully get warmed depending on load and ambient temps.