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Stop-start systems: saving gas at the traffic lights; BMW 118d

by David Zatz

A stop-start system shuts the engine off when the car or truck stops, and restarts it instantly when the brake or clutch pedal is released. The radio, gauges, fan, and other accessories keep going while the engine is off.

The first Chrysler vehicle to use a stop-start system (other than the Durango/Aspen hybrid) is the 2014 Ram 1500 HFE. It is, though, soon to be available on the 2015 Chrysler 200 four-cylinder and standard on the 2015 Jeep Cherokee V6.

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The Ram's starter, with 300,000 tested on/off cycles, has stronger flywheel teeth and a special starter solenoid. The battery is 800 amps, and the alternator capacity is 220 amps. If the battery's charge is low, the truck will stop using the stop-start system.

(See the 2013 Ram 1500 powertrain and feature walkthrough.)

As (say) the crankshaft begins to turn, the oil still on the metal parts is built into a wedge of liquid, creating pressure which lifts the metal crankshaft off the bearings. Higher surface tensions result in faster buildup of the wedge, which is one reason why synthetic oils are superior in many cases; they are engineered for the higher surface tension.

Lubrication is not a serious issue, because oil clings onto metal parts for a long time (due to their shearing strength) - any hybrid is proof of this. No accumulator or secondary oil pump is needed. Cooling is another story, especially with turbocharged cars, but that's handled by the computer. If the oil temperature (as it leaves the turbocharger bearing cartridge) is hot enough that the oil might "coke," or have carbon particulates drop out, the stop-start won't activate. Synthetic oils' sheer shear strength and higher coking temperature helps, so some engines with stop-start systems require synthetic oil.

Bob Lincoln wrote: "I've been in Florida in August, 100° F and 90% relative humidity. You can [shut off the compressor] for 60 seconds. At a red light, turn off the air conditioner compressor [not all cars let you do this], but keep the blower fan running. You will still get some cooling the whole time."

Climate control is also an issue, though both heat and air conditioning do continue to work for a while with the engine off; the heater core stays hot and the evaporator stays cold after the engine is off. Cars used to routinely shut off the compressor to avoid freezing the evaporator. With stop-start systems, the fan keeps blowing air through the evaporator or heater core; the computer can restart the engine if the interior starts to get too cold or hot.
It is unlikely that Chrysler would produce a stop-start system that doesn't take hot and cold weather into account.

Electric compressors are possible, but are likely to be used only on range-extended electric cars where the gasoline engine is normally off.

Chrysler 200 / Jeep Cherokee system

The Chrysler 200's owner's manual says that stop-start is enabled after the customer drives over 5 mph at least once. To go into "autostop" mode, the system must be ready, the car must come to a complete stop with the shifter in Drive or Neutral, and the driver must keep the brake pedal down. Then the engine shuts off, and an indicator lights up. The climate control may change airflow automatically.

The system does not work if the driver is not buckled in or has their door open; if the battery is too hot or cold; if the car is on a steep grade; if the cabin is too hot or cold; if the defroster is on, with the fan on High; if the battery is not fully charged, the engine is not warm enough, the hood is open, maximum a/c is set, or if the brake pedal is not being fully depressed.

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The engine restarts automatically when the brake is released or the accelerator is engaged. The Hill Holder keeps the brakes on until the transmission is engaged.

Fuel mileage savings, with other changes, are up to 2 mpg combined in the Cherokee V6, without the need for massive batteries or heavy motors at the wheels. Thanks, Mike V.

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The stop-start system of the 2015 Jeep Cherokee, which helped boost combined mileage ratings of some models by 2 mpg, was developed by Chrysler itself, according to a company spokesman. "Chrysler Group powertrain engineering did all the integration and controls for this system." The components are, since Chrysler has not made starters for some years, purchased from multiple suppliers.

The Jeep Cherokee uses an enhanced-starter system, whose main components are:

  • A stronger, 220-amp alternator
  • An absorbent glass mat (AGM) high-durability battery
  • A modified starter with hardened gears, reinforced castings, higher-power motor, longer life brushes, and higher-precision bearings and shaft
  • A large Denso starter solenoid
  • A voltage stabilization module that prevent voltage dips during cranking, providing a buffer for the car computers and such
  • A Bosch battery sensor which predicts voltage and capacity, and measures alternator current and battery temperature
  • Special programming in the car computer

If you want the details of what it's doing, there's a trip computer setting that tells you the status - things like whether it's off because the engine is too cold, or the battery is low, or the cabin temperature is too off (or whether it's active).

The lack of vibration and engine sound is distracting for some, but may be something people get used to and ignore after a while. In our test, we never encountered a situation where stop-start got in the way. The engine started almost immediately when our foot left the brake pedal, and only once did we hit the gas before the engine was really ready, resulting in a slight chug but otherwise no problems.

While a belt starter-generator (BSG) stop-start setup is reportedly still being developed, since it is seen as the most efficient system, it remains in the future. The BSG setup was referenced in both public and private Chrysler materials.

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Trying the stop-start system of the BMW 118d SE

Stop-start systems are already becoming common in Europe. With this in mind, on a recent trip to Britain, we rented a stop-start equipped car.

The way BMW handles stop-start in its 1-series diesel, with the six-speed manual transmission, is sensible and similar to Fiat's method. The computer makes sure there's enough juice in the battery, that it's warm enough outside and in the engine, and such; and if the car comes to a stop (or is braking so it's near a stop) and the clutch pedal is up, with the stick in neutral, the engine cuts out. The computer appears to keep heating and cooling in mind as well, not activating stop-start until the interior is cool (or warm).

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Chrysler is expected to use a belt-starter-generator (BSG) system with these features. In cars with an automatic transmission, the system uses the brake pedal, rather than the clutch, as a trigger to restart the engine.

When the light turns green - or, in England and Wales, when it turns yellow-and-red - the driver pushes down on the clutch to shift into first, and the engine restarts immediately. We never had any hesitation in the engine, despite numerous stop signs and traffic lights, and it never slowed us down. What's more, the system works just fine in stop-and-go traffic, starting up instantly each time. I could not beat it to first gear - by the time my foot was letting out the clutch, the engine was running. I'm sure a faster driver, or one more practiced with right-hand-drive perhaps, could beat it, but only by trying.

One can shut off the system in the BMW by pressing a button; it seemed to be programmed to shut off for a single stop (or, rather, to keep the engine running) if you hit the clutch pedal, then released it, as I admit to sometimes doing as "driver error" (thinking traffic was about to move because someone ahead pulled forward six inches).

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My wife disliked the stop-start, finding it freaky and unexpected. I liked it, particularly as the vibrations of the diesel engine ended at each stop. I also liked the higher mileage; city traffic didn't seem to have nearly its usual cost.

As for the rest of the car, the handling was excellent; the ride, fairly comfortable but definitely biased for road-feel over comfort. The BMW dealt with bumps, jiggles, and cobblestones quite well, all things considered. Getting in was another story, with a tight entrance into both the front and rear seats. The seats themselves were not well cushioned, though they were well bolstered. The interior is fairly tight for four, though quite liveable, and there is cargo space enough for four moderately sized suitcases... if you take the privacy cover off.

The BMW 118d puts out 143 hp and 236lb-ft of torque; with a six-speed manual transmission, it has a rated combined fuel use of 4.1 liters/100km (57 mpg).

Acceleration was good for a little diesel, but not thrilling. Economy ranged from 40-50 mpg (US). Shifting was easy, partly thanks to the even power of the twin-turbo diesel; the clutch was smooth and had a good active range.

It was odd to drive a car in this price class without a built in compass, though there was room enough in the iDrive system for a full owner's manual. Fiat has a nice compass in their 500L, along with a similar number of personalization options, though their system is easily as hard to use as the iDrive.

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The personalization features lag far behind Chrysler's, and seem to match those of the 1999 Jeep Grand Cherokee, one of the first vehicles to have personalization through menus rather than "press the lock button twice while in Accessory mode." iDrive is awkward in a RHD car, and too far back for easy access; Chrysler's steering wheel mounted controls are more convenient. The irony is that BMW is indeed known for being driver-oriented, but in this case, they really aren't.

The trip computer was good and clear, - and there were actually two of them, one which gave the departure time, trip duration, distance, fuel mileage, and speed; and other which just gave distance-to-empty, fuel economy, and average speed.

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There were some other oddities, including a difficult push into Reverse, and a "touch for three flashes" turn signal behavior (acceptable because you could shut it off - unlike similar behavior in Chrysler cars). The car also had an intensely annoying second stop for leaving the signals on; instead of having a simple mechanical setup, pushing the turn signal lever past the first click left it on until it was shut off by pushing it again, rather than by knocking it physically in the other direction. Doing the latter turns on the turn signal on the other side for three flashes. Like the 2011-14 Charger V6 and 300 V6 automatic shifter, this is a system that shoves being electronic into your face, defeating its main purpose.

One nice feature was the cruise-control-replacement, a speed limiter, almost essential in the UK with its speed cameras everywhere; unlike a cruise control, it sets an upper limit, while the driver can go slower if needed. Set it to a speed and you can't go any faster. It's easy and quick to change, either in 1 mph or 5 mph increments.

The stop-start system struck me as something one gets used to, over time, and extremely sensible for those who do a lot of city traffic, commuter-crawls, or who simply have to wait at long Bath and Cardiff traffic lights.

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