a car question.

[quakerboy]

I have recently had the opportunity to drive a new prius. Watching the gauges and meters and things, it seems to me that at least half the milage improvement that it offers seems to come from not having the engine running while sitting at a stoplight. In fact I have known people who turned their cars off at predictably long lights. This made me wonder...

Why would one need the electric motor and extra batteries to achieve that savings? Why couldn't you make a car that doesn't run the engine when you are not moving, and only kicks it back on when you press the go pedal? But without the battery and electric motor. Thus reducing complexity of the vehicle but still saving gas.

[Rochester]

1. I remember reading on "Car Guys" in the paper awhile back...

...that the amount of gas used to start the engine is about the same as the amount of gas needed to idle it for 3 minutes. So in a traditional car you won't save any gas unless they are exceptionally long red lights if this is accurate. Then there's wear and tear on the starter. Don't know how the Prius handles that. But I wouldn't want to abuse my starter like that.

[quakerboy]

2. Thats pretty close to what I had always heard

My dad had said it was closer to 5 minutes.

But the Prius (and I assume other hybrids) must have some workaround, because I notice that the engine is on and off all the time, particularly at stops.

Which is what sparked my thinking on this. If there is some workaround for the problem, why couldn't we apply it to other cars? Even if it only made a 5 mpg difference, that would still add up.

[NuclearDem]

3. Hybrid engines start differently than regular gasoline engines

The Prius uses one of its electric motors to start the car and get it going initially. In a lot of cases, it may or may not even use its internal combustion engine to power the vehicle, and so gas may not even be used at all.

This is why you'll only see this phenomenon with hybrid cars. Pure internal combustion engines have only the gasoline-powered engine to power the car; the hybrid can use its electric motors to start and initially drive the car and not use any gas at all.

The only way you'll be able to reduce gas consumption is when the internal combustion engine is completely off. Hybrid engines can do that and run purely on electric power, which the Prius will do at stop lights or while waiting at a drive-thru. Often times, you won't even have to turn the Prius off; it'll automatically shut off the gas engine when the computer says the electric motor can handle the drive.

So, in short, it doesn't make a difference in gasoline engines whether you turn the car off or not; you'll be using about the same amount of gas either way, because it takes a lot more fuel to actually get the motor running than you would save by not letting it idle at stop lights. Neither does it make a difference in hybrids whether you manually shut the car off; the computer will handle engine management.

[quakerboy]

6. my question goes to why, perhaps.

The prius I have been using turns the gas engine off at stop lights. When I hit the gas it turns it back on within a short time of starting movement. If I am careful to accelerate very slowly, I can keep the gas engine off longer. But even if I am not and I just step on it so as not to annoy all the drivers behind, the car still gets considerably better mileage than my own compact car.


So why is it that the prius can shut the gas off at the light, even though it comes right back on when I start movement, where it would be ineficient to do so in a standard car?

[NuclearDem]

25. Because in addition to the acceleration you'll need to do to move from the light in a normal car

You also have the initial gulp of fuel the engine takes in after starting up (notice when you turn the car on, right after it turns over you hear a VROOOOOOOOOM out of the engine...and the RPM temporarily kicks up to between 1500 and 2000). From what I understand, the Prius doesn't gulp fuel that way when the computer turns the gas engine on.

That initial gulp the engine takes is to provide enough force and mechanical power to move all of the various parts in an engine; to get the crankshaft turning and the pistons moving. Because the electric motor is already providing a significant amount of mechanical power to the driveshaft, the gas engine in the Prius doesn't have to gulp fuel to power the vehicle.

[HopeHoops]

4. Post #1 is correct - it is generally more efficient to let it idle than to stop and start it.

[EOTE]

7. I don't believe that's true.

Of course, it would depend on how much idling is done, but I think there is only a very brief increase in the amount of fuel consumed when the car is started. Much like the very brief (milliseconds) increase in power consumption when you first turn on the light. There are a number of new, high end cars (like the current 7 series BMW) that do shut the engine off while idling and turn it back on when resuming in order to preserve fuel economy.

[HopeHoops]

9. Even with fuel injection, it is still true. It was MUCH more the case with carburetors.

A big block V-8 could suck in 15 minutes of idle juice just firing up! Fuel injection is a lot more efficient, but it still takes some priming to get the damn thing started and there's enough of that to run it for a while in an idle. The three minute rule is pretty accurate for most cars.

[EOTE]

10. I'd very much like to see some hard data regarding that.

Starting can be hard on your starter, that's why vehicles with such a system typically have a heavier duty starter/alternator integrated into one unit. These can handle the additional stress of the increased stops/starts. However, the additional fuel used in the start is miniscule compared to the fuel saved when you would have otherwise been idling. I guarantee you that I know quite a bit about this stuff.

http://en.wikipedia.org/wiki/Start-stop_system

On edit: I guess you should be telling BMW that they're wasting their money with all their fancy research. The 5-10% increase in fuel economy must be all in their heads.

[HopeHoops]

11. BMW is the definition of wasting money - as in buying one.

Ugly damn things, too.

[EOTE]

12. So you got nothing.

Like it or not, BMW has been on the forefront of many automotive technological advances. They also make some of the most fuel efficient high performance engines available (the current 328i gets about the same fuel economy as a 4 cylinder Mazda3 with about 60 additional hp). Regardless of your opinion of BMW (and Audi and VW and Citroen and Fiat and Alfa Romeo, all of whom have start-stop systems), the technology exists and it DOES provide a fairly substantial fuel economy boost.

[HopeHoops]

16. Yeah, and they also built the faulty engine on the Airbus. Read on...

The start-stop system is different. I'll give you that and it is a major improvement. But without it, the rule is still "let it idle" and most cars don't have it.

And shit, the last of the companies you mentioned to make anything even remotely attractive was Alfa Romeo and that was over 30 years ago!!! Technology is great, but if it is wrapped in a dog turd package it just isn't right. Now Porche has got the gig - technology and style that can't be beat, well, other than that ugly-ass SUV-like thing they came out with. Somebody must have been drunk when that was approved for production.

[EOTE]

17. Ahh, so you've gone from "It don't work" to "It works, but it's ugly!".

I guess that's a bit of an improvement. All of these cars look a metric fuck-ton better than the Prius, and it doesn't seem like you have a problem with the system there. Baby steps, I guess.

[HopeHoops]

20. Hey, so I wasn't up to speed on the start-stop system. I'll admit that. And as for the Prius...

It looks like something that was scribbled on a sheet of toilet paper during a "religious moment" after Thursday's "all you can eat" burrito night at the "Sushi TexMex Emporium". Very few cars have any style these days, but some are just butt-ugly.

[snooper2]

23. Edmunds says you are wrong as well...

http://www.edmunds.com/fuel-economy/we-test-the-tips.html#test6


Side note...
If you don't think this is a sweet ride, you may want to get that checked.

[HopeHoops]

24. I'll put that silver thing in the "yawn" department. Boring.

[MicaelS]

13. No stop / start for me, unless there is an override

I live in North Texas, and in the summertime the heat index can reach 120F. I do not want I want my engine stopping, and the A/C with it. No cooling, and too much wear and tear on the A/C Compressor, especially the clutch. I will take the hit on mileage, my comfort is much more important.

[EOTE]

14. The systems available now are rather advanced.

The technology has been around for almost 30 years now. The more advanced ones have an integrated starter/alternator so all of your belt driven accessories continue to run while your engine does not. Most of the systems now are so seamless, you don't even know they're there.

[MicaelS]

15. So, you're telling me my A/C will be running off my battery?

That is one hell of a load for a single battery. Unless we're going to start having multiple lead-acid batteries, and I'm not sure I want that, either.

[EOTE]

18. A lot of them use some form of regenerative braking to ensure that juice never gets too low.

Believe me, they've thought these systems out very well. The battery life in these systems is typically a bit longer than standard systems utilizing a conventional starter and alternator. If there's not enough juice to run accessories from the ISG and battery, the system won't enable until it's at a more acceptable level, but it's incredibly rare for that to happen.

[MicaelS]

21. Well that sounds better, but I still would want an override..

I'm stubborn about things like that.

[EOTE]

22. Understandable.

That's one of the reasons that I'll never drive a car with an automatic transmission ever again (or those crappy manumatics either). But direct mechanical linkages to the things which control our cars are becoming more and more a thing of the past. I had driven my past car (a 2001 VW GTI) for several months before I realized that it had a throttle-by-wire system. I actually had to read about it without figuring it out on my own because the system was so seamless. Now, so many cars not only have throttle-by-wire systems, but brake-by-wire and steer-by-wire as well. Last year, Toyota did a good job of bringing the potential problems with such systems to light.

[LawnKorn]

26. Old carburetors had a device called an accelerator pump that sprayed gasoline into the throttle bore

Cars with carburetors had an all analog system that would spray a quantity of gasoline directly into the venturi every time the throttle was opened (weather it was needed or not). The accelerator pump was the cause of 'flooding' an engine when the gas peddle was repeatedly depressed while the engine was stopped.

Most all modern engines are fuel injected today, and the computerized engine controls only allow the fuel injectors to spray in the exact amount of gas needed to run the engine. Tamping on the gas peddle of a stopped engine these days does not result in flooding.

Rochester Quadrajet
Carter AFB
750 Holley Double Pumper
45 Weber six pack

[mucifer]

5. I have a 2007 prius I really like it . But the milage isn't very good during the

winter in Chicago if I make quick trips which is what I do most of the time as a home health nurse. If I'm in the car a long time The mileage is better. I do save LOTS of money on the brakes. I used to replace my brakes every year with my corolla. In 3 and a half years so far I have not needed to do any work on the brakes. (That is a common prius thing).

[EOTE]

8. This technology has existed for quite a while.

http://en.wikipedia.org/wiki/Start-stop_system

[Deep13]

19. A few things...

electric motors are more efficient than gas engines

the motors generate electricity that is stored in batteries when coasting or braking

having an engine that is used only when needed to recharge the batteries allows that engine to be small and to run at peak efficiency where the throttle changes on a gas engine causes great inefficiency then changing speed