Poor fuel economy over short distances

[Bataleon]

Originally Posted by Daox

Its automatically controlled, every engine does it. A warm engine is an efficient engine. Fuel vaporizes properly with a hot combustion chamber, components expand with heat, and many other things. Engines need to be warm to be efficient.

Thanks Daox, I thought this might be the case

Another reason to avoid short trips I guess.

[redpoint5]

A warm engine is indeed an efficient engine, but it's my understanding that high-idle is primarily used to get the catalytic converter up to operating temperature quickly, for emissions reasons.

There are people on the Prius forum that spoof the ECU into thinking the engine is already at operating temperature. Not only is the fast-idle eliminated, but the engine is then allowed to shut off at stops, which it normally doesn't do until operating temperature is reached.

I don't know if spoofing the temperature sensor would cause premature wear on components like the cat, or just how awful the emissions are for the environment, but it would certainly reduce overall fuel consumption. I'd be tempted to perform this mod, but so far I've just shortened the amount of time it takes to warm up the engine by grill blocking.

[Bataleon]

Originally Posted by redpoint5

A warm engine is indeed an efficient engine, but it's my understanding that high-idle is primarily used to get the catalytic converter up to operating temperature quickly, for emissions reasons.

There are people on the Prius forum that spoof the ECU into thinking the engine is already at operating temperature. Not only is the fast-idle eliminated, but the engine is then allowed to shut off at stops, which it normally doesn't do until operating temperature is reached.

I don't know if spoofing the temperature sensor would cause premature wear on components like the cat, or just how awful the emissions are for the environment, but it would certainly reduce overall fuel consumption. I'd be tempted to perform this mod, but so far I've just shortened the amount of time it takes to warm up the engine by grill blocking.

Excellent reply, thanks for the info.

Does the grill block cause your engine to run much hotter than usual? Or is it still within its safe operating range?

[RedDevil]

One effect I noticed from blocking my grill is that the engine stays warmer for longer when parked. Even after 4 hours the engine is noticeably warmer than ambient.
Simply because it more or less closes off the engine bay so the warmed air in the engine bay cannot escape that easily

Warmup times are indeed faster - but not by much.
You need to measure the temperature. I already had to reduce some grill blocking.

[redpoint5]

I only have the lower grill blocked on the Prius, and on typical drives the coolant temperature stays at the normal temperature of around 195 F. On some long and steep hill climbs I've seen the temperature rise to just above 200 F, which is still within the normal operating range. Above 210 F I would remove some grill blocking, but so far this hasn't happened. Most of the grill is blocked on the TSX and that temperature stays around 188 F.

[Focus-Ak]

Originally Posted by Bataleon

... When I start my engine it idles at 1200 rpm until the coolant reaches 140°F (60°C) after which the revs drop to 850 rpm. At the same time the fuel flow rate drops from 40cc/min to 20cc/min.

So at the high idle you are running about 0.63 gallons/hour, which seems about right.

With the help of a custom tuner I was able to get the idle on my Focus (notorious high idlers) down to 600-700 RPM within a very short block from my house, instead of 1200-1400 for the first 20 minutes or so and a then drop about 900-1000, which was the lowest.

So now coasting in neutral from the end of the first block I am consuming 0.33 gal/hr, and within a minute later I'm down to 0.25. Fully warmed up I'm idling at 0.21-0.23 instead of formerly about 0.35. Also, the engine now drops to low idle in about half the time it previously did (3 sec vs. 5 to 7), which makes the car more predictable and drivable.

On my UltraGauge I'm seeing much better MPG on short trips even from a cold start; probably a 25% improvement on those -- 20 vs. 15 or 16 on one route; 26 vs. at best 19 on another which is mostly coasting. Just my sense of it; no practical way to do A-B-A testing on this. I'm still in early trials on this.

[Isaac Zackary]

They make 1600W and even 2000W circulating block heaters. Put one of those on and plug it in 1/2 hour before starting. Don't idle it. Heat the cat up by driving it.

[jcp123]

It's also to light off the catalytic converter and to get the oxygen sensors warm enough to be able to read the oxygen stream.

Short trips hurt no matter what, though a block heater would help immensely. Nope, there's nothing wrong with your car.

I wish I had such a low cold idle. I'm at 1500-1800rpm at first start, and it'll gradually taper off until normal operating temperature. At 120*F coolant temp, mine is still idling at a full 1300rpm, and even at 165*F I'm still at around 1000. Get over 180, and it finally falls to its ambient 800rpm (~650 in D).

[Focus-Ak]

Originally Posted by jcp123

It's also to ... get the oxygen sensors warm enough to be able to read the oxygen stream.

The Focus has two heated O2 sensors (narrow band); they warm up fast. My UltraGauge says I'm already in closed loop a few feet out of the driveway (cold start, and go with no idling warm-up). That was the case before my custom tune, as well as now.

Before starting I have the ignition switch in the run position while buckle-up, release the hand brake, etc. That may help with some extra time for the O2 sensors to warm up a bit.

[jcp123]

True. Modern cars tend to have heated O2s; my Echo is closed loop within 30sec usually. Don't know how universal this is though.