The VW Jetta I rented was exactly like you describe, with a turbocharged four-banger and an 8-speed transmission. But I found it great fun to drive and quite enjoyed the engine sound, although if I actually owned it, I’d worry about the stress and reliability of the engine over the long term. I believe there are are two engine options on the Jetta and from the feel of this one, it probably had the larger 2.0 liter one. My present car is about the same size and for the long term I appreciate its 2.7 liter 6-cylinder engine, which has been totally reliable for years, always super smooth, and never sounds stressed. I believe it’s just a three-speed transmission with overdrive, but the overdrive is a major feature for fuel economy and reduced engine wear.
About starting by just firing a plug/knowing which cylinder was ready. How long does the compression hold in the cylinder? I would think that the pressure would leak out just sitting there and have no compression when fired.
The only problem I foresee, and I know I’ll catch flak for this, but on cold mornings I want to start my car and let it run for five or ten minutes to warm up. It allows me to enter the vehicle without being miserable, it lets me clear the windshield with a brush instead of a scraper (or even just a flick of the wipers), and as far as I’m concerned it isn’t inefficient at all. For those few minutes I don’t want the car to take me anywhere, I want it to be a space heater, and space heaters are always 100% efficient. MPG can wait until I’m on the road.
I’ve never driven a car that shuts off automatically, but as long as I can override that feature until the car is warm, I’m okay with it. Otherwise, we’ll have a problem.
You shouldn’t have to override it for that reason. This feature only works when the engine is warm, so it’s not going to be shutting off while you’re warming it up.
Indeed. By that logic the world’s most reliable car should be something like a bare bones 1955 VW Beetle. After all, things like automatic transmissions, power steering, power windows, air conditioning, heaters, radios, liquid cooling, etc. are all just more things that can potentially break.
I used to have a engine monitor called DashHawk that would display various engine parameters like boost/vacuum, intake air temperature, and engine load. IIRC an 2008 Mazdaspeed3 would take about 30% engine load to just idle in neutral. In a modern fuel-injected computer-controlled engine, idling at a stoplight actually burns far more fuel than coasting downhill or driving on a level surface at constant speed.
I’m not talking about warming up the engine. That only takes a minute or two anyway. I’m talking about warming up the passenger cabin and windshields when the outside temp is below freezing.
[Emphasis added]
This isn’t true at all. Respectfully, you may have misunderstand what “load” means in this context—it’s not the same thing as power.
When your engine idles, it feeds in enough fuel and air to keep the engine at about 1000-1400 RPM while still overcoming friction and driving the alternator and any other belt-driven accessories.
When you’re driving down a flat highway at 55 mph, your engine has to do all that and overcome wind and rolling resistance.
Combustion efficiency varies with RPM, and most four-cycle and Miller-cycle engines have significant pumping losses at small throttle openings. And while those losses are large on a relative basis, the power required to idle is low enough that the pumping losses are small in absolute terms.
It definitely takes more fuel to drive at a steady speed on flat ground than it does to idle.
This is…implausible.
Well, the miles-per-gallon is certainly worse.
Stop-start was introduced as a way for the vehicle manufacturers to comply with increasingly strict emissions regulations for their vehicles.
With regard to fuel consumption when stopping and restarting a carburetted engine, here’s my interpretation; the fuel delivery in a carb relies on a vacuum being produced in the venturi. This vacuum is produced by the induction within the engine (the pistons on the ‘induction’ stroke moving down within the bore on a reciprocating-piston engine) and the speed with which is produced is not instant. Whilst it is being built up, some fuel will be being drawn into the engine, which can potentially lead to a slightly ‘rich’ mixture (hence wasted fuel). I don’t come to this with a super-scientific authority - just that I know how the system works.
Engine design nowadays is all about marginal gains - moving over to fuel injection has given us the major leaps forward in terms of emission reduction & fuel economy which is great for the environment and also our wallet (in terms of mpg).
I have had many turbocharged cars that use the stop-start technology and i have noticed that when coming off a fast motorway and pulling up to stop at a junction, the S/S system will happily shut the engine down with no regard to the speed that the turbocharger will still be spinning at. Turning off the engine will starve the turbocharger of lubrication and wear it out. Seems loopy, right? I concluded that the manufacturer would rather comply with the emissions regulations on the new vehicle rather than be overly-concerned with ‘protecting’ the service life of the turbo. Turbos can be replaced easily. Emission legislation can’t. Some modern S/S vehicles will not operate the S/S for a while after start-up to optimise the battery recharge; likewise, if the climate control settings are set to hot (for winter) the S/S system with prioritse ‘thermal comfort’ for the driver and passengers.
Following on from that, the starter motors on modern vehicles without S/S system are actually pretty tiddly and are only designed to turn the engine over to start. They can’t handle being used for extended periods (i.e. cranking the engine for ages if it wont start). Modern engines should start on the button, full stop. So stopping and starting your non-S/S vehicle persistently will not do othe starter motor OR the battery any favours, because it is quite simply not designed to be used in this way. S/S vehicles have more robust starters and batteries to cope with the extra demand.
The initial pressure in an unfired cylinder at engine stop won’t be very high, for a couple of reasons:
#1: The engine is typically at idle already when the shutdown occurs; under idle conditions, i.e. with the throttle plate at max restriction, the manifold vacuum is high, so the TDC compression pressure won’t be very high when shutdown occurs. Typical idle manifold vacuum in an idling spark ignited engine is about 20 inHg.
#2: The engine can’t stop with a piston at exactly TDC; if it did, then it would have no leverage to get the engine spinning again solely from a combustion event. It has to stop significantly after TDC, meaning the cylinder volume at shutdown will be a little higher than at TDC.
I played around with this thermodynamic process calculator a bit. I found that for a manifold vacuum of 20 inHg and an engine compression ratio of 13:1, and a stopping point just after TDC such that the cylinder volume was three time the TDC volume, the cylinder pressure was just 38.3 psi. That’s an absolute pressure, meaning the gauge pressure would only be about 24 psi. Not much pressure to leak down. If the stopping point is such that the cylinder volume is 6 times the TDC volume, then the cylinder pressure is exactly equal to ambient; there won’t be any leakdown.
If the cylinder has any pressure above ambient when it stops, it will leak down from there to ambient pressure in fairly short order, probably well under a minute. But if you’ve ever played with a hairspray-powered potato cannon, then you know that igniting an ambient-pressure flammable mixture can still produce plenty of force. This is also true for the engine: even if the cylinder only contains an ambient-pressure mixture, lighting it off can still be enough to get the engine spinning.
There’s no emission concern due to the leakage, either. unburned mixture that leaks out past the piston rings ends up in the crankcase, and the crankcase vent routes it back to the intake tract, where it gets drawn into the engine and burned anyway.
This kind of system won’t work on a diesel engine. For ignition, diesels rely on having a mass of air in the cylinder that’s been heated scorching-hot due to the high compression. Once you stop the engine, and that compressed air bleeds away, any fuel you inject isn’t going to ignite. A start-stop system on a diesel will need some other mechanism to get the engine spinning again before resuming fuel injection.
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I think those cars have water-cooled turbochargers with electrical water pumps that keeps running even when the engine is stopped. Not sure about oil, but why couldn’t (and wouldn’t) they do the same for the oil pump?
On mine (BMW M2) the turbocharger is liquid cooled and the pump is electric, keeps going when engine is off if necessary. Oil pump is driven off the engine.
However the identifiable problem with car turbochargers in the past if you shut them down suddenly was heat, very hot ‘hot section’ of the turbocharger would ‘cook’ the oil. That’s avoided by water cooled turbo w/ electric motor pump.
The other post posited a somewhat different issue, that shutting down when the turbo is still spinning would eventually damage the turbocharger bearings because no oil pressure (oil will tend to continue to siphon through but not under pressure). People give this as reason (among others) on BMW owner forums to turn off start-stop, which on most models you have to do each time you drive unless you get the car re-coded.
People (virtually never design engineers at the firm in question, or at all) say this on the internet IOW. Is it really true? I’m skeptical. BMW owners manuals do have a disclaimer about start-stop, “Certain vehicle components may experience additional wear as a result of this system” but some people read a lot into such legalese disclaimers, a large % of car owner manuals now consist of them.
Also the car is being driven very lightly compared to its capability in conditions where start stop activates much. I’m not start-stopping on high mountain pass roads or Pacific Coast Hwy. I’m hard on tires in that kind of driving, wore down my first set of summer rear tires in under 10k miles, but not really on the engine, flooring it only for matters of seconds to get past people quickly on two lane roads. Nothing much compared to people who take the car to the track (though again there’d be a disclaimer there also about the impact of that on component life). I doubt start-stopping is doing anything material to my turbocharger’s life.
These cars will not engage in start/stop behavior when in Park, or when remote started.
Maybe it depends on speed, but I definitely saw my engine load% lower at times when driving than when idling. I assumed it was because of the added efficiency that momentum added. Maybe it was more when I was going downhill, even slightly so.
The original statement said a couple of times ‘level ground’. As stated the engine is overcoming some rolling and air resistance whenever it’s moving, isn’t when standing still.
In the ‘eco’ driving mode my former BMW’s transmission would go into neutral if you lifted you foot from the gas going downhill, so engine operation almost the same as idling at a stop light (with the start-stop function turned off, obviously), car idles at same ~600rpm, same fuel consumption per hour, car rolling freely, give or take a little more load on the power steering pump etc. That would seem to be basically the limit, equal fuel consumption rolling in neutral or standing in neutral. If the car remains in gear downhill as most do, the ‘load’ is less because now the wheels are driving the engine at higher than idle RPM, but the setting on the fuel injectors is still ‘idle’, not cut off* and fuel consumption per hr still probably higher because the injectors are actuating more times per hour.
*some 8 cyl car/trucks cut off fuel to 4 cylinders when not needed, but also do that when standing idling (against besides start-stop feature).
In my experience, GM pioneered start/stop in the mid 1980s. I had an '84 Buick Regal that happily shut down at every stop sign or red light. The start function involved a lot of cranking the starter, cussing wildly, and pounding the steering wheel. I’m glad they perfected the restart function. It was rather annoying.
In addition to what Corry El said on this subject, a remote unmanned fuel system can be stuck anywhere, too. Filling one is a lot less difficult than stocking and manning a convenience store. I used to buy biodiesel from an unmanned station, and there’s lots of unmanned fuel stations out there that are used by professional freight systems. I used to live by one that also offered propane back in the 80s, and I regularly drove by another on my way to work for years. Theft of liquid fuel from underground is really slow unless you’ve brought your own electric pump. I’ve hand pumped 20 or so gallons of diesel from a drum before. It’s not quick.
Most of the time, fuel is kind of a loss leader for the convenience store that most people (or truckers) fuel up at. If they’re making a profit on the fuel, it’s usually because they’re at a premium location. Most of the profit any fuel stop makes is from the food, beer and other miscellaneous items it sells in addition to the fuel they have to offer. Gotta keep the humans going, at least for the time being, and the machines run for longer than the average driver between stops. I’d say that an EV driver probably plans it out more than an ICE but the top end of either of those will easily get most people from breakfast to lunch. So, the profitable part of the shipping would probably keep happening, the loss leader would probably just be slowly shifted to the electric bill as the pumps become less profitable than the chargers and aren’t replaced. Then, the issue becomes one of whether the site will upgrade their electrical capacity or just stop offering fuel or fast charging and move on to other businesses like other places that used to be gas stations in my area of the world.
I’d imagine that as a probably being a slow process. Even at this point, I’d expect the tanks at the recent QT expansion in my area to last for 20 years or so.