I agree.
In one case near my house where I could use engine braking, the difference in this 55 MPH zone is 55 MPH with engine braking or 61 gliding in Neutral.
I’m comfortable exceeding the speed limit by 6 MPH there; it’s both safe and draws no risk from law enforcement in this place.
However, on lengthy downhills I’ve seen on the W.Va turnpike, the difference between engine braking and gliding is… 65 MPH vs 85 MPH. That risks both points on your license and a fiery death at the bottom of a ravine. Bad idea.
Based on my experiences with manually killing the engine at stop lights, in the range of 5% in a suburban commute and perhaps as high as 15% on urban surface streets.
I suspect that the 15% number might go a little higher if you had a lengthy commute on surface streets with a ton of lights, but fervently hope I never have a commute like that.
ETA:
By the way, the system you’re suggesting WOULD cause a lag getting the vehicle moving, no matter what. Even the best instant-start non-hybrid with today’s technology would give you no less than a 1/3rd second lag before you had access to acceleration.
http://www.mazda.com/mazdaspirit/env/engine/siss.html
I am drawing on a hazy memory here, but I believe variable-cylinder engines just shut off the valve train for unused cylinders. The original engineering on such engines was more complex, but after a generation or two of experimentation it was discovered that just leaving the cylinder sealed (both valves closed) was almost as efficient and far less problematic than trying to partially vent it to reduce energy lost to compression.
Probably not enough to make the complexity worthwhile. Hybrids have the inbuilt advantage (and drawback) of very complex and sophisticated engine and powertrain control. Adding this to single-engine cars would probably not result in useful reduction of fuel usage. As this whole thread has pointed out, modern engines with EFI can control fuel to the droplet and are already optimized to use as little fuel at idle as possible. A normal driver might save a gallon a month using a shutoff/restart system, and that’s probably below the noise level for fuel economy at present.
Well, the problem isn’t if you put the pedal down lightly, while engine breaking. I’m sure those of us who are old enough (or drive old manual cars) have stalled with the clutch out, restarting it by dropping the clutch, and know what that feels like. The problem is if you cut fuel delivery long enough, you’ll just dog it when you try to accelerate. No big deal, but I bet you’re right nobody tried killing fuel when there’s no demand. Instead it would reduce fuel injection due to decreased intake manifold pressure and low throttle angle.
Not sure what you mean by that, but injectors and spark happen on a schedule. Of course, the schedule depends on operating parameters, but in general, when demand goes up, things get recalculated, and if the results appear before the next cylinder event needed to be scheduled, it happens. Otherwise it happens next cycle.
I think the trick is in the transmission.
Based on watching my mileage when I just start and stop my engine, I doubt I’d save anything by shutting off at stop lights. Perhaps my 2000 Chrysler minivan isn’t the most efficient starter. It is very effective birth control, though.
Little energy is lost to compression in the end, since most of it is regained on expansion. The biggest difference would be the increased friction on the crankshaft bearing. A crankshaft is a pretty darn good bearing. The simple design sounds smartest to me, especially if the valve closedown happens after an exhaust cycle.
I don’t know much about these variable cylinder designs, but I do remember the engineers at Ford Sci Lab discussing the the Cadillac 8/6/4 engine. I think that was the first to try this, and had issues.
How are you watching your mileage when you’re stopping and starting the engine?
Any time you are at idle your spot [instant] MPG is 0.0.
You mean the before and after MPG?
Well as we’ve discussed, you don’t have that fine control of the fuel with a throttle body, injected or carbeurated. So ignition without fuel control doesn’t lead to an instantly running engine, or at least not running smoothly. The injection system can match the fuel to the RPMs and load pretty closely, so any cylinder can just start injecting again without any noticeable feedback from the engine. It’s not like push starting car where you release the clutch and get a serious kick from the engine braking, but with a throttle body it just won’t start firing evenly right off the bat, the throttle body has to get saturated with a good fuel air mix first.
NitroPress, interesting note about keeping the valves shut. Sounds like the kind of thing a mechanic figured out while the engineers were still scratching their heads.
You may be more on target than you suspect. I recall from the sports-car days of the 1940s and 50s that the fix for a broken valve follower or tappet was to remove both followers from that cylinder and leave it sealed - small engines in particular would run better with a completely missing cylinder than with one messing up intake and exhaust flow. It was also a quick field fix in a time when anyone who drove such temperamental beasts needed a box full of them to complete a day’s jaunt.
Some grizzled tech with experience in that era might have said, “Hey, mister shiny-buttons. On and off is good enough, here!”
If I’m going down a hill and my tachometer is reading, say 2,000 RPM, shouldn’t shifting into neutral still keep the engine at 2,000 RPM if the wheels are driving the transmission which is turning the engine?
So, why does my engine drop to idle speed, about 500 RPM. If the wheels and transmission were continuing to drive the engine, it would still read 2,000. If the fuel injectors cut off completely, then why am I reading 500 RPM, the same as idle mode.
Because in neutral the wheels are no longer driving the engine, the engine has been disconnected from the drivetrain. We’ve been talking about coasting in drive, not neutral.
I got to see the trick of the computer cutting fuel to the engine while coasting in gear on a recent road trip. For some reason the instantaneous fuel economy on the trip computer was set to liters per 100 kilometers (L/100km) despite being in the US. There were sufficiently long descents on the highway that the trip computer would show 0.0 L/100KM. That didn’t happen every time I took my foot off the gas, but it did happen anytime I was off the gas, in gear, the engine was sufficiently above idle, and that situation remained in place for a sufficient amount of time.
Once I figured out which buttons on the PS/3 controller disguised as a steering wheel needed to be pressed to show miles per gallon, the in gear coasting reading switched to 99.9 MPG.
Start stop is trivial on a hybrid because they don’t need the engine to get going, the electric motor can do it. It’s important to understand that hybrids like the Prius and Chevy Volt don’t have regular transmissions per se, not even “CVTs” although many people call them that. The transmission is more akin to a planetary center differential you would find in a Subaru WRX Sti, except running in reverse so that instead of taking one source of torque and splitting it between 2 axles, it takes 2 sources of torque and blends it into one output shaft, while at the same time also approportioning torque between the 2 sources as well, so that the gas engine can charge the battery by running the electric motor, or vice versa. It’s a “CVT” in that it doesn’t have any gears to change, all “gear changes” are really just the differential shunting torque between the 2 sources and the output to achieve the desired acceleration.
It’s really neat and a very high tech solution to a problem that is much more complex than people think and a big reason why the Prius gets such good mileage.
To throw another wrench into the thing, a lot of cars now come with gas engines featuring direct fuel injection. The new Ford/Mazda engines using start/stop actually just start the engine by reading the cam position and firing the appropriate injectors and sparkplugs, without any need or the starter.
Start/Stop technology is very common in Europe, due to European fuel economy standards that recognize its benefits. The American EPA and NHTSA mileage cycle does not benefit from stop/start, it doesn’t count towards the manufacturers CAFE score, so American market cars usually don’t have them unless they are hybrids. The first non-Hybrid car in the US that had it was the Porsche Panamera from 2 years ago, presumably because Porsche’s volumes were so small that it was cheaper to just sell one drivetrain globally instead of engineering a seperate, shittier car just for America. There has since been a few more, mostly big Audis I think. The European regulatory system has its own problems, the principle one I can see is that they only measure the fuel economy of new cars, and say nothing about how well this start/stop thing has to work 100,000 miles down the road, or how long the battery/starter system has to function. A car where the battery can no longer sustain the start/stop function will simply shut it off and just act like a regular car, with the owner non-the-wiser.
I actually have a relevant point to make here and that is until recently even the European start/stop systems were only implemented on manual transmission or dual clutch (DSG) gearbox models. The reason is that on a conventional torque converter automatic transmission, the lubricating oils in the tranny and TC are kept flowing by an engine driven pump. When you shut the engine down you also shut down the transmission oil pump and the tranny is no longer getting oil. Now start the engine up again and the tranny and converter has to suddenly shift into gear and start spinning before the oil has been fully lubricated. This is why shifting into neutral and shutting the car off while the car is coasting (as some more adventurous people do) is a really terrible idea, because the transmission is still spinning at speed with no oil, and doing so at stop lights is also a bad idea especially since those horrible Chrysler minivan trannies were under-spec-ed to begin with.
The newest automatic transmissions, like the ZF 8 speeds that the new RWD Chryslers have, actually use an Accusump style device in the transmission to keep it lubricated while start/stop is being used. The new Audis and BMWs that have start/stop all use this transmission.
Seriously you don’t understand the functional difference between neutral and drive?
Wow.
I’m pretty sure I do. How does an engine with no fuel maintain an idle speed of 500 RPM?
This has been bothering me, too. I think there are some assumptions about automatic-trans vehicles here that may be limited to modern, limited-slip torque converters and clutch packs.
Not all vehicles with automatic transmissions will force the engine to matching RPMs when coasting down a hill in drive. It may be true of most newer cars with very tight (even locked) couplings, but I’ve driven many an auto whose engine would drop considerably in RPMs if you coasted down a long grade while in drive. Until the era of the very limited slip, lockup torque converter, the converter would not drive the engine anywhere nearly as well as the reverse, normal engine-to-tranny drive.
NM.
After re-reading I see that we are in fact talking about shutting off fuel while in gear, not in neutral. I somehow missed this important fact.
Isn’t this because once you take your foot off of the accelerator, the transmission upshifts to the tallest gear that won’t put the engine below idle RPM?
Using a conventional automatic none of this stuff will work. Going fast enough downhill you can get a car with an automatic to start up, but it has to go must faster than when you pop the clutch on a standard, and it’s kind of unreliable. I don’t know if newer automatics make a difference.
IIRC his thread on Southern accents, he’s from either Europe or Eastern Europe.
English isn’t his first language, so factor that into understanding him.
Is Nitro’s post you responded to talking about Neutral gear Engine Off Coasting as your response implies?
I’m thinking he’s talking about coasting down in gear, based on how he’s writing.
I thought he was Canadian. Who else would be a Mapleleafs fan?