Poor fuel economy over short distances
 

Hi all,

Most days I make a two mile trip (each way) to campus and back in my manual 2006 1400cc Volkswagen Polo (2200 lbs curb weight).

Yesterday I measured my fuel economy during the two mile trip home (using Torque App) and got an average reading of 19.4 mpg(US). This morning I averaged 28.3 mpg(US), slightly better because it is downhill most of the way.

These figures are pretty poor considering:

1. It's a small car with a small, relatively modern engine
2. The ambient temperature was around 72F
3. Tires were pumped to 44 psi
4. I was hypermiling where I could.

Do you think there's something wrong with my car?

Thanks in advance.

Probably nothing wrong with the car, just the commute. It's too short to properly warm up the vehicle. I'd ride a bicycle for such a short distance.

Good advice. I'm going to start using my bicycle a lot more now.

My wife drives It in town only. It seldom drives further than 3 miles in one go.
She is not into hypermiling (to put it mildly) but I doubt it would have a lot of effect anyway.
Check out It's fuel log...

In absolute terms It does not use a lot of fuel though. The tpg (trips per gallon) rating is outstanding.

Interesting figures, thanks RedDevil.

Use of a block / coolant heater would help some in a vehicle that does short trips, even in warm ambient temperatures. Of course that only helps with coolant temp. Efficiency is still going to suffer from other cold drivetrain components like the transmission, tires, etc.

Yup all cars get terrible gas mileage the first few miles. Modern cars have some insulation for the fluids and thermal management to warm things up faster (so your second trip takes less warm up time) but you still need to warm it up the first time.

I thought Diesel_Dave was heating up the transmission pan, oil pan and coolant before he started his truck (With pad heaters?) though. It helped him.

Interesting idea.

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.

Why is this? It seems like an unnecessary waste of fuel considering the ambient temperature is around 72°F (22°C).

Is there anyway to disable this or is it automatically controlled by the vehicle's ECU?

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.

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?

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.

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.

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.

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.

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).

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.

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

Older thread but everything is still relevant; I've tried to search to see if anyone else is doing this and haven't seen anything.

I have a 4 mile commute; in the morning I can use the block heater at home but I have no place to plug in at the open parking lot at work.
So, I've been trying to use the sun and my brain to give the engine temperature a little boost by drawing the heat inside the car into the coolant.

When I park the car after I get to work, I set:
The HVAC controls to Recirculate so the only air going over the heater core is from the sun warmed interior.
Vents to Bi-Level to encourage air flow through the heater core as the car interior warms in the sun.
The temperature to Hot so that the heated water from the heater core will flow into the engine block (but the thermostat won't let it go to the radiator).
I also set the fan to High so that when I start the car, the heat of the interior is immediately transferred into the coolant with maximum effect.
When I get in the car and turn the ignition on 9.5 hours after I parked it, I'm seeing coolant temperatures 5 to 10 degrees above the exterior temperature. Not a huge increase but any improvement is an improvement.
As I drive, I watch the coolant temperature increase and as soon as it increases over what I feel the interior temperature is (maybe I should put a thermometer in the cabin to be more precicse) I move the temperature control to full Cold and adjust the other controls as needed for personal comfort until the engine is fully warm.

It probably helps that I have an older car with cable actuated HVAC controls; cars with vacuum actuated HVAC systems may benefit from setting the controls in the last moment before shutting the engine off? Also, some vehicles will automatically engage the A/C compressor with certain HVAC settings; they may need to change the settings before starting the vehicle.

One added benefit as we move into the warmer spring and summer months is that I'm pulling heat out of the cabin and moving it into the engine throughout the day.

That's smart! Setting the heater to max when the interior is hot while the engine is still cold actually cools the interior.
I'm going to try that out for sure, see if the engine temp does rise faster like that (on my UltraGauge). If I ever get to drive again :)

about as good as my V8 6.0L hybrid 20.5 in the city :thumbup:

Interesting idea! I've thought about routing inside air to the intake so it'll suck in warm air. That should help the car to not need to run as rich during startup, in theory anyhow.

Except for the large current battery requirements, there are 12v powered heat units in my F250: fuel bowl, glow plugs. In the ranger, I spliced in a solar panel 1000 watt 12v water heater into the coolant line since the electric motor doesn't have water cooling.

Yes, assuming your car has an Intake Air Temperature Sensor.

I honestly never thought of doing this, reverse thermosiphon to pull heat out of the cabin.

I wonder if a solar power muffin fan pulling air through the system while parked would help?