Engine warm-up tricks

The technical challenge is to get the engine heater to 70C, the threshold that allows 'hybrid mode' operation where the engine shuts off when it can.

On the first, cold-soak start, it took six minutes to reach the hybrid threshold. During this time, the engine runs constantly burning gas the whole time. So there are multiple approaches to shorten this interval:

• block heater
• thermistor spoof
• driving the warm-up control laws
• air inlet block

As pointed out, the 1.5L engine has a port on the top of the engine block on the firewall side for a block heater:

It is the second most difficult place to reach either from the top or bottom. I found it a a little easier to put the car on ramps and reach up to insert. Just put the thermal grease on one side so air can escape out the other. It is a miserable job but once done, lace it and plug-in when needed.

My experience is about 30-45 minutes on mornings with temperatures of 40F or lower, saves about about 1-2 minutes of the 5-6 minutes of warm-up. It is significant but above 40F, the effects rapidly fall off. It does heat the top of the engine block but not hot enough for the car to drive off in 'hybrid mode.'

BTW, I also tested a transaxle heater but found they were not reliable. The wires soon broke so I don't fool with them any more. The advantage is a warmer transaxle has lower, internal friction, less drag.

Another approach is to 'spoof' the thermistor so it reads 70C. The problem is spoofing before it reaches 35-40C can lead to engine stalling out. However, there have been at least three such spoofs:

1. diode, resistor, switch - when the blue coolant light goes out, flip the switch, but remember to turn off when you park.
2. transistor circuit - I don't have the schematic but one waits for the 40C point and then drives a latch-up to shift it to 70C. It would be easy enough to tap the coolant lamp and switch in the spoof drive transistor.
3. microcontroller - my approach, NHW11 Prius Temperature Hack, but others have done the same with common microcontrollers.

The third approach is to realize keeping the engine 'off loaded' as much as possible during the warm-up AND rolling works almost as well. So in the winter, I change my route to drive through the neighborhood:

So when I back out the driveway, reverse turns off the engine. Then I turn to descend, slowly a shallow grade with the shifter in "N". Cutting through the 25 mph neighborhood, I take every opportunity to shift into "N" so the engine runs in idle most of the time. But there is a less well understood warm-up, the catalytic converter 'light off.'

During the first 50 seconds, the car does everything it can to use traction battery power instead of engine power. I have accelerated to 35-40 mph and seen the ending, instant MPG reading nearly 50 MPG. Then shifting into "N", it pegs the MPG. The engine still has to complete the 4-5 minute warm-up cycle and shifting into "N" during this time absolutely minimizes the engine load and fuel burn. The trick is to park near the parking lot exit so the car can get on the cross street on traction battery in the 50 second window.

The most important infornmation is to know the current engine coolant temperature and that takes some sort of OBS scanner or tapping into the thermistor circuit. Either works and then you can make informed driving decisions.

Don't forget to put an air inlet block in the front fender. Because I was not driving high speeds, I kept my split, water noodle in all summer and had no problems with over heating. But I was using my wife's 2010 on the A/C days this summer. In earlier years, I keep the radiator inlet block in until the coolant temperature reaches 92-94F and then toss it in the trunk until the Fall.

GOOD LUCK!
Bob Wilson

 

This is specific to our 2003 Prius. The 2010 has much better warm-up control laws. Although we never had one, even the NHW20 is better than our 2003.

Bon Wilson