My theory on why some Gen 2s consume more oil after 150k miles

Accumulated mpg.
Mix of freeway driving vs. city driving is probably 60% / 40%.
I believe it helps the mpg to have relatively warm ambient temps.
When I lived in California the indicated mpg was in the low 40s. I think that part of the problem was the California formulation fuel had lower energy content (and yet cost much more) than the fuel available in Arizona.

 

I don't know why people have this misconception. The HV battery never gets "exhausted" doing motorway work. I have driven many thousand motorway miles in the UK (doing 70-75 mph) and Europe (doing 110-130 km/h) and never once experienced this phenomenon of "battery exhaustion" In fact, quite the opposite. My traction battery was continuously on the 6-7 bar range. As an aside, I got some of my best consumption (as in lowest) stats in Europe, so go figure.

 

I also must agree with the above.
Although I am now retired and rarely use the motorways, my car was my company car and as such was used every day on the motorways of GB at a continuous 70 to 80 mph for the first 2 years of it's life. During this time the car clocked up 60,000 miles with the display showing 6 or 7 bars of charge and no oil use with 0w20 oil.
The engine now at well over 100,000 miles still shows no sign of oil use between oil changes at 10,000 miles, although it is checked at every refill of fuel. The battery level indicator still shows an almost constant high level of charge apart from when the engine first starts from cold, the display dropping to 3 blue bars as the battery reduces the emissions on the cold start. Fuel consumption is at around 60 mpg in cold weather improving by at a minimum of 5 mpg in warmer weather.

John (Britprius)

 

Hi there dolj,

Maybe I didn't really explain what I meant clearly enough when I mentioned HV Battery "exhaustion".

As I understand it, the HV battery only holds enough energy to drive the car for a few kilometers by itself. Even if the car was able to run with the ICE off at 70mph (which it isn't), it wouldn't get far. The battery would be "exhausted", by which I mean, discharged to the point where the HSD says it is empty (which as we know is still about 50-60% charged, but is at the minimum charge level for optimum battery longevity). To all intents and purposes, as far as getting extra power out of it into the Electric motors, it's discharged.

Even if it is only assisting the ICE (let's say, providing 10% of the total power), it's still not going to be able to assist for more than 10 or 20km at 70mph. From that point on, the ICE has to (a) supply ALL the power to drive the vehicle, and (b) provide additional power to recharge the HV battery.

This is assuming that you are continuing to drive at a constant 70mph, which is fairly realistic. Sure, there will be undulations, and from watching my own MFD, I can say that at higher speeds, the HV battery energy flow is continually changing. Charge, discharge, charge, discharge.

This is because the Toyota engineers didn't design the HSD to run the battery out before recharging it. The system is obviously taking every opportunity to optimise power utilization, with its rapid cycling from charge to discharge, maintaining the battery SOCat a level where it has reserve power to assist the ICE, but also has reserve charge "room" to accept a hefty regen charge, resulting from a downhill run or a decel to stop. In other words, at an appropriate midpoint SOC.

My point is that, at a constant 70mph, on average the HV battery is not assisting the ICE at all. On average, it is actually causing the ICE extra work, due to the inefficiencies in converting from mechanical energy to electrical energy to battery charge, which itself is an exothermic process.

That's why I suggested that at (constant) motorway speeds, having a car that has regen cannot be expected to help the economy of the ICE. Which is why I'm very surprised at Patrick's reported figures. But, as he says, that is accumulated mileage.

Partly, I was surprised because watching my own G2 MFD instantaneous economy figures (and 5 minute bars) at motorway speeds, they seem to be consistently lower than my accumulated average figure, which is what I'd expect.

I still think that an "identical" Prius with all its regen equipment stripped out (and geared to produce the same ICE RPM at 70mph), would be more economical at 70mph, than a normal Prius.

Hence, I think that the engine wear is going to be higher, and the oil consumption higher as well in this scenario, than it would in an equally efficient engine (ordinary car) without the regen gear.

 

An interesting thought experiment. A concern that I have with your analysis is that it is not possible with current automobile technology to produce an "identical" Prius without regen.

The Atkinson cycle gasoline engine cannot perform at low RPM, it lacks sufficient torque to move the car at low speeds. MG2 provides the low-end power train torque.

 

Yes, absolutely true. In order to perform this experiment one would have to accelerate the regen-less car to 20 or 30mph by some other means, such as a convenient hill, or a pusher vehicle. After that, it should be able to accelerate to 70mph under its own power. One would also have to fix the MG1 shaft, and alter the gearing somewhere so that the ICE was running at the same RPM as it would be at 70mph in a normal Prius. Of course, it wouldn't be identical at all, I was just meaning the engines would need to be as close to identical in performance and efficiency as possible, in order to "prove" the experiment.

Yes, of course. That's why it needs MG1, which is capable of running backwards but holding torque, to allow the ICE to drive the vehicle at low speeds. Otherwise, it would be very like trying to accelerate a manual transmission vehicle from stationary, in 3rd or 4th gear.

Actually, this raises an interesting (but off-topic) question. Presumably, it must be possible, using only variable intake valve timing, to fully control an Atkinson cycle engine's power - without the need for any throttle whatsoever? This would completely eliminate pumping losses, and make the engine even more efficient.

Does anyone know if that's ever been done?

 

In order to eliminate pumping losses you would have to achieve a full atmosphere of pressure in the cylinder at the beginning of the compression stroke. This is not possible at low loads with a spark-ignition engine since there is a specific ratio of fuel to air to allow combustion initiation with a spark. So a full atmosphere of air requires a relatively large quantity of fuel. This is essentially the condition for maximum torque output. You can operate an engine with "excess" air if you select a compression ratio high enough to heat the air to ignite injected fuel. This is a diesel. Some manufactures do offer gasoline engines with variable valve lift, e.g., BMW's VANOS system. This just increases intake velocity to better atomise fuel. Gasoline engines with direct injection (e.g., Mazda SkyActiv) may also reduce pumping losses by operating with a air/fuel ratio that is leaner than otherwise attainable.

 

Hi there hlunde,

Maybe you and I have different definitions of the term "pumping losses"? Or maybe you don't understand the Atkinson cycle? In my understanding, pumping losses occur in a petrol (gasoline) due to the need to restrict airflow into the intake manifold by way of a butterfly valve. This is where the term "throttle" originated. The engine is throttled, literally meaning "starved" of fuel-air mixture, to regulate its speed and power.

As a result, because the fuel-air mixture is still generating power within the engine (and the engine "wants" to accelerate), the pressure in the intake manifold decreases substantially to "hold it back", hence the use of vacuum gauges to observe manifold vacuum by "boy racers" and the like. This intake manifold vacuum is where pumping losses originate. [The only time pumping losses not not occur in a petrol engine is at WOT (Wide Open Throttle), which is why fuel economy racers (of old?) only used bursts of power at WOT with the engine off in between.]

The intake manifold pressure may be 0.25 atmospheres or even lower at idle, and lower still on deceleration. However, the exhaust outlet pressure is always atmospheric (1 atmosphere). Therefore, the engine is effectively an air (vacuum) pump, sucking air in from 0.25 atm at the intake port and discharging it at 1.0 atm. (Hence, engine braking). It has to "pump" the air going into the intake ports up in pressure by 0.75atm. Ok, so the combustion process does that, but it's still a loss, that is always present at partial loads. Just imagine how much energy would be required to continually turn that engine at those same pressure differences, with no fuel being injected - so it's purely acting as a vacuum pump - and you will more or less be able to isolate the pumping losses, if you can subtract mechanical friction.

[You may be aware that diesel engines without intake throttle valves (i.e. most of them) have very little engine braking (and that mechanical fuel-injector-pump equipped engines require a (typically mechanical) speed limiting governor to prevent them "running away" and over-revving to destruction)? There is no effective engine braking because, of the full cylinder of air that gets compressed each compression stroke, 99% is still present as the (un-fueled) "combustion" i.e. expansion stroke occurs, so there are almost no pumping losses in "open intake" style diesel engine. This is why diesel engines are fitted with compression (Jacob's, "Jakes" or other brand) "Exhaust" brakes, to give them some effective engine braking by massively increasing the engine pumping losses, from atmospheric at the intake ports, to some much higher pressure at the exhaust ports. This pressure-energy is, of course, then wasted in that characteristic Exhaust Brake noise...

The whole point of my post was that, with the Atkinson cycle engine, as the inlet valve is closed late in the compression phase, it is theoretically possible to eliminate the throttle plate. The (modified) Atkinson cycle engine achieves an increase in engine efficiency by reducing the compression stroke length in proportion to the combustion stroke length, by "late" closing of the intake valve. The effect of this "lengthening" of the combustion stroke is the ability to extract more energy from the burning fuel mixture (before "popping" the exhaust valve as in a standard engine, and releasing a whole lot of useful temperature-pressure-energy down the exhaust pipe). [You may also be aware of the different sound of a diesel exhaust compared to a petrol one, as less energy is wasted in the diesel exhaust. The Atkinson cycle engine exhaust sound must be even quieter!]

Continuing, compression only commences once the intake valve has closed. This does not require any alteration of the fuel mixture away from stoichiometric. If the valve timing could be altered to close from "0" to almost 180 degrees in the compression stroke, it would be possible to leave just enough atmospheric pressure air-fuel mixture in the cylinder at the very end of the compression stroke, to produce just enough power to maintain the engine at idle (or at any other desired speed / power setting). Surely, this would be highly efficient!?

Doing that, I submit, would entirely eliminate pumping losses in the petrol engine. My question is, has it been done yet?

 

BMW Technology Guide : Valvetronic

 

I don't think that we disagree about the nature of pumping losses. You elaborations may be helpful to others.
Your suggestion about closing the intake valve very late in the compression is interesting. You would still need a full atmosphere of pressure in the cylinder at this point, but the air mass would be less and of course the required fuel delivery would be commensurate. So thermally the engine would operate as a much smaller engine. One trade-off would be having the combustion of a smaller engine together with the mechanical friction of a larger one. Another observation is that this would not be especially advantageous in a hybrid where the engine already operates intermittently at prescribed conditions of relative efficiency. For a non-hybrid, Atkinson cycle engines tend to be physically larger/heavier for a given output. So one might want to look at a trade-off against a smaller direct injection turbocharged engine. Finally, to answer you question about how far anyone has gone with quasi Atkinson development -- I don't know and cannot readily find any references to this.

 

Good work Patrick!

So it has not only been done, it's being sold... Excellent.

My concern was that it would cause a rough idling engine, but I guess not.

 

Hi again,

My proposal was to control engine power using valve timing (as BMW have evidently already done), to eliminate pumping losses, not to make it run that way all the time! So I figure the friction factor would stay much the same as in an engine that it throttled by - well - a throttle! I guess it's up to the designer to decide whether it's an Atkinson cycle engine, or a variable valve timed engine which runs much of the time in Atkinson mode...

 

In retrospect I agree that all engines operating at part throttle have high friction relative to the power output, not just pseudo-Atkinson cycle engines. But another thought has occurred. If you considerably prolong the closure of the intake valve during the compression stroke, then you are operating with a very low compression ratio, which in the ideal Otto cycle results in much-reduced thermal efficiency. I've done a brief search for technical papers on the modified-Atkinson cycle and the earliest that I find is one from the Israel Institute of Technology in 1982. Of course there may be earlier ones. I have found more-recent papers published by Mazda, VAG and GM --- but none by Toyota. The IIT paper is here:

The Otto-Atkinson Engine - A New Concept in Automotive Economy

And while we are really off-topic now, I will offer a quote that I read in one of the above papers. "The Atkinson engine has an 8-to-1 ratio (or less depending upon intake valve closing.) This significantly reduces the sealing ring friction". This could be on- topic since it may suggest that Atkinson cycle engine have less ring pressure that other contemporary engines.

 

Hi hlunde. This is true for the Otto cycle but doesn't apply to the Atkinson cycle engine. Technically it's not the compression ratio that influences thermal efficiency, it's the expansion ratio. This is actually true for both the Otto and Atkinson cycles, in each case it's expansion ratio (not compression ratio) that's important. It's just that for a regular Otto cycle engine the two terms are synonymous, so understandably people only talk about compression ratios.

 

Hi again hlunde,

Yes, but isn't this exactly the same in the Otto cycle engine, when it's "sucking a low intake manifold pressure"? Effectively, the compression ratio is much lower, and the cylinder pressure probably doesn't even reach atmospheric until the piston is about 75% of the way through the compression stroke. I don't really see the difference - the issue being that with stoichiometric mixtures, you have to reduce the cylinder fuel/air volume to get the engine to idle. Direct injection would be the way to go.

Anyway, not sure which Prius engines (but maybe all) have a theoretical compression ratio of 13:1, which forces them to always run in Atkinson cycle mode, and produces greater thermal efficiency from both sources.

Great, thanks, will take a look later.

AFAIK, low piston ring pressure is not a required feature of the Atkinson cycle design; it's merely to reduce friction. But I do believe the Prius engines have low ring pressures as well.

 

Nice re-discovery. I talked about low tension rings on the page 2 of this very thread.
My theory on why some Gen 2s consume more oil after 150k miles | Page 2 | PriusChat

 

There are two ways that a piston ring is forced against the cylinder wall. One is by the spring force of the ring; low tension rings have less spring force and typically less piston ring width (less surface area against the cylinder wall). But rings are also forced against the cylinder wall by gas pressure and the paper cited above suggests that Atkinson engines operate with lower gas pressures. Does this affect oil consumption in high-mileage Prius engines?

 

IIRC, prius compression is 10:1 while expansion is 13:1, so the above quote doesn't apply to prius.