Why Engine efficiency (BSFC) worse during warmup?

What can we do about it other than keeping the engine warm? I've been looking at engine heaters, not sure which one is good. Some are $40 and some are $50. I would also need a timer to start the heater about an hour before I go to work since I probably wouldn't want it to run all night. If other family members needed to use the Prius, they would have to remember to unplug the heater before they take off. The more I think about it, the less enthusiastic I get.

 

Track "long term fuel trim" and engine coolant temperature.

Bob Wilson

 

These are very good questions and I'm waiting for someone more qualified than myself to answer them fully.
However, I always thought cold engine requires rich mixture and that results in incomplete combustion and low efficiency.
Then there is increased friction due to oil viscosity, denser air, and fuel/water condensation on cylinder liners.
Finally, the combustion temp is lower due to excessive cooling, resulting in lower efficiency.

I googled and found this Prius-specific Canadian study on warmup fuel loses that is a good read (even if doesn't answer all the questions):
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/veh_sys_sim/vss050_jehlik_2011_o.pdf

 

My testing with the 1.5L engine and a spoofing circuit revealed the coolant must be 40C and above or the engine will 'stall out.'

Within seconds, the cylinder wall and piston temperatures are high enough there are no condensation issues.

What is missing from the charts is the role of transaxle oil warm-up on engine-to-wheel loss. But these were part of my earliest Prius studies:
2003 Prius - Cold Weather and Transaxle

Everything they show on pp. 8 is consistent with the transmission warm-up.

My notes: 03 Prius Transaxle Temperatures

One key requirements in the marathon driving stunt was to increase each commute duration from ~20 minutes to over 60 minutes. This ensured the last half of each commute had a warmed transmission allowing 99.9 MPG over that part. The first 20 minutes would typically provide ~70-75 MPG but 40 minutes later, it was often pegged at 95-99 MPG for the whole trip.

Bob Wilson

 

Yes regarding aluminum alloy pistons, but no regarding steel liners surrounded by cold coolant. I you were right, we would not have cold start cylinder wear or fuel dilution in oil due to cold idling.

Yes, but the OP talks about ENGINE efficiency.

 

My observations are that below ~60C, the engine control laws use fuel to warm-up the engine. This may have changed in the more recent models and it would be a good thing if they did. Also, oil viscosity changes with temperature so a cooler engine will have more internal, lubrication drag.

Bob Wilson

 

I found one abstract from an SAE paper that claims 10% for a small, air-cooled Honda engine. But my subsequent attempts to find any SAE credible papers has not worked.

I typically use a short-cut, the starter power. With a fast recording OBD scanner, you can get usable metrics for our Prius.

Bob Wilson

 

I also have been watching BSFC on my Android. During warm up the engine is not working under high load, it produces only 50 Nm of torque, that alone gives us a BSFC (from the cart) of around 300 even if the engine would be fully warmed up. Try to go over 50 mph and give gas just a little under the center of HSI (maybe it's even SOC dependent), you will get a similar condition - small torque, low rpm, high instant MPG, but BSFC will read around 300 g/kWh.

I rarely see 220 mark, the number is usually bouncing between 220-240 when fully warmed up, and around 240-250 when the coolant is not fully warmed up. Changing the engine speed will momentarily drop this figures out of this range as the input data (fuel flow, rpm, torque) is not synchronized. So I don't believe that your observation of how fast one varies the position of hsd, can be read correctly, but it may be true.

 

I have read SAE papers and the warm up stage advanced timing was chosen to generate max heat to bring up the exhaust cat converter temp (to 600 deg F).

When you gave it high throttle (more than the power HV battery could fill), Prius would cancel the warm up timing to provide "emergency" power.

My guess is, that gives you better BSFC but without Cat at working temp, more pollution would sprew out. That can't be good for the life of the Cat conv.

My suggestion is, let the car do it's thing. It is good to understand and monitor it through Torque app though.

 

There is one problem, even if BSFC is better it's still not as good as a warmed up engine and during high acceleration the fuel flow will be higher, so a small deviation from optimum BSFC may mean same thing if you are babying the throttle. The other problem is that because of traffic you can not accelerate very long, so this pulse will be short and then you are again stuck with normal warming up consumption.

But all that said I do use this technique if I must accelerate to a higher speed and maintain that speed. I think it's still better than taking it slow and completely drain the battery.

 

Sounds good.

Once you get a hang of BSFC, monitor the MG1 Power. If it is generating electricity, you know that amount is splitting through the series path.

You want to minimize that since it has to go through conversion loss. It usually happens when you accelerate heavy at low speed or heavy passing on highway. This way, you maximize ICE direct power to the wheels.

 

MG1 can spin as high was 10,000 rpm but if it isn't creating any resistance (negative torque), it won't be generating any electricity.

PSD splits 28% of the torque from ICE to MG1 and 72% to the wheels. That does not change. The change in torque adjusts the power generation or consumption.

MG1 can consume electricity. Not only when it starts the ICE as a starter but also at higher speed. MG1 actually spins backward in order to achieve "overdrive", resulting in slow ICE rpm at high vehicle speed. That's also when MG1 can become motor and MG2 generator. Their roles are reversed.

Yes, that's part of it. There are losses in the inverter to match the voltage (both are AC). There are losses in generator and motor as well.

If the electricity needs to go through the battery, there will be further loss as AC needs to be converted to DC and voltage has to drop to ~202V. Plus there are losses in battery charging and discharging.

In the order of desirable to less desirable, 1) ICE direct mechanical power to the wheels. 2) Series hybrid path -- eCVT torque multiplication through electric motor. 3) Power flowing through the battery.

Basically, the entire system needs to be considered other than BFSC of the ICE.