April 22, 2012 7:38 AM Subscribe
I do have one suggestion though that has worked for me:
Whenever you come up to a stoplight (that you know is going to be relatively long) shut off the engine. This works.
2009 RAV4 Limited V6 (mostly driving in the city)
21.2 avg MPG
22.5 last MPG
25.9 best MPG (vacation trip out west)
137 fuel ups
This a good tip, however, MOST drivers/passengers don't feel comfortable turning off their engines at stoplights. This is why most manufacturers (even as basic as Kia) are starting to implement the Stop and Go feature in their cars which turns off the engine at stoplights and automatically turns it on again once the brakes are released.
Good tip, but I partially disagree. Over the last two tanks in my 2012 Hyundai Accent I have been testing out some hypermiling techniques to squeeze out some better mileage. Most notably, I have focusing on coasting as much as possible and driving as if I don't have brakes. The result is a significant increase in mpg, I'm beating EPA estimates in city and mixed driving.
I don't use your tip a lot, unless I'm at a very long light. It certainly helps though. Thanks for sharing.
Aside from hypermiling techniques, there are lots of modifications you can do to increase the fuel economy of a vehicle, for example:
1.) Reduce the weight of the vehicle by removing non essential items. Every pound of vehicle weight requires a small amount of energy from fuel to move it around, so if you reduce the weight, then you also reduce the amount of fuel needed to move the vehicle.
2.) Fit tyres with a low rolling resistance, or increase tyre pressures, both reduce the energy losses due to friction between the tyres and road surface. For example, an ice skater can move very fast without using much energy because there is very little friction between the skates and the ice.
3.) Improve the vehicle's coefficient of drag (cd) by removing spoilers or making other aerodynamic improvements, this minimises energy losses due to aerodynamic drag. This is why high speed vehicles are aerodynamic, to reduce the energy needed to maintain any given speed.
4.) Change the final drive ratio or transmission to a taller gear set. Many vehicles share common components or have a closeley related vehicle with which a transmission swap is possible. This reduces the engine RPM at any given speed, increasing fuel economy.
>MPG is built in to any given vehicle. The engineers ... have
>designed your automobile to certain specs. Nothing you do will change
I would say that while it is difficult to significantly increase MPG via mods (there are examples above), it is all too easy to decrease MPG (low pressure in the tyres, towing, roofrack, overloading, a coked-up engine).
Depending on the car, shutting off the engine at red lights will keep your car in open loop for most of your trip. For all you know, you have a car that comes out of open loop after a mandatory 3 min wait and you hit a light ever 5 mins.
Open loop 3/5ths of your trip? No thank you!
@MMUK Although there are other reasons for my recent increase in MPG (change in fuel blend, weather, change to synthetic oil), you can see on my Fuelly profile that the last 2-3 tanks I've had were a large difference from my previous tanks. The recent change for the better is when I started using some hypermiling techniques. Again, I won't attribute it all to hypermiling, but it certainly made a pretty good difference.
Ah, yeah, but note that I said 'via mods' :-) Hypermiling will improve your MPG much more than any physical change to the vehicle. Congratulations on your new driving style.
It wasn't until I bought a scangauge that I found that turning off the engine at long stop lights is actually not very effective at all.
Modern vehicles just use very little gas at warm idle. If the original poster were to idle his Rav4 for a full hour, it would reduce his mileage over a full 10-gallon tank of gas by less than one mile per gallon. When you consider that part of the benefit of stopping the engine at red lights is offset by the extra fuel needed to restart the motor, the benefit goes down even further.
Unfortunately I do not have a good figure for how much fuel is needed to restart a hot engine (both in the richer starting AFRs, as well as the energy from the battery). So I can't say for sure where the break-even point is, only that the total benefit is very small. The scangauge is useful for remarkably few things. But at least we know it is nonzero.
I'm not so sure that a modern fuel-injected engine uses that much more fuel at startup than an extended idle. Back in the day when you had to press on the accelerator while starting the car, that may have been true. It would be interesting if someone had real data on what the difference was, however.
BTW, the corollary to that is that if your car is relatively modern (made after about 1985), and fuel injected, you should never press the accelerator when starting it.
There is a difference between "not very effective" and "counterproductive", and I think that turning off a modern car at long traffic lights falls in the "not very effective" category as opposed to the "counterproductive" - there's a reason why some car makers are putting the auto-shutoff switches in cars for reducing idle gas usage - it's "free" to do (other than some additional load on the starter motor) and nets a positive (though not significant) uptick in overall Fuel Economy.
The breakeven point is said to be between 10 seconds & 30 seconds (depending on the engine).
Turning off the engine at stop lights may work OK with petrol engines in warm climates but here in the UK where temperatures are colder and more people drive diesels it can cause problems.
The amp draw required to restart a big diesel engine even when warm is significant due to far higher compression ratio. Constant use of the starter can lead to flat or damaged battery and damaged starters.
The vehicles here that are fitted with stop/start are often fitted with uprated starters, larger or even twin batteries and run lower compression diesels. For example BMW have 16:1 compression in their latest diesels compared with 20:1 or even 22:1 compression ratio on older diesels like my own. Even then there have been numerous issues reported with failures of the systems.
Also if your vehicle is used mostly for longer journeys (extra urban/rural/motorway) the additional weight of the stop/start systems larger batteries combined with less torque at lower rpm can actually lower your overall mpg. So it is not something that works for everybody.
Even though you do not press the gas peddle, the ECM automatically richens the mixture to restart the engine. Most ECM have a lookup table that looks up the coolant temp and air temp to come up with an increased pulse width to richen it up for starting. There is also an afterstart enrichment and a decay setting to control how long the enrichment lasts. Direct injected cars would need the least amount of startup and afterstart enrichment. It's not much, but I don't think it's worth shutting off unless you're going to sit more than 3 or 4 minutes.
Shutting off the engine has the side effect of shutting off the heater or air conditioner. Depending on outside weather conditions, this might be unpleasant. If the car has automatic headlights, shutting off the engine probably also turns off the lights.