At 18:05, 28 November 2008 (PST) Brian said …
Great job. There are a few things to take into consideration. The lower the voltage (which is true during start), the wider the pulse width to the injector. This wider pulse width does not correspond to more fuel. The fuel injector reacts to power input. At lower voltages, you need a wider pulse to get the same power (electrical power) to the injector. The conclusion from this statement is that although the pulses are higher during crank, the amount of fuel is not as high as the pulse width would lead you to believe. There is more, but a lower percentage more. Which lowers the 7 second time even more. Great job again with nothing but a laptop as your data collection equipment.

At 14:40, 30 November 2008 (PST) Rob said …
Hi Brian, Thanks for the comment. That’s a very good point. For a 7 second period I think the effect would be relatively insignificant since the voltage will be high for most of the time, but the fuel volume estimated during cranking (when the voltage drops significantly) will be overestimated. For more accurate results I would have to calculate the volume of fuel based on both the pulse width and the voltage (pulse height). The voltage should only affect the amount of time it takes the injector to open and once the injector is open, the fuel flow should be independent of voltage. So a better formula might be (Volume = A*PW – B*V – C) where PW is the pulse width, V is the voltage and A, B and C are constants. Calibration would be required to determine A, B and C (ie I would run the injector with different dummy signals with known values of PW and V and see how much fuel is expelled in a given time). Something to consider for future experimentation. The data may also be available somewhere already.

At 11:08, 22 January 2009 (PST) Greg said …
I think the logic that less fuel is flowing when injector voltage is lower is mostly faulty. The injector is an on – off valve. It is not proportional in any way. Pulse width modulation performs flow control. The only issue in question is whether or not the valve takes significantly longer to open with reduced voltage under starter load. There are two types of injectors and the current controlled type injectors would probably operate exactly the same during starting as the do normally.

At 18:12, 3 February 2009 (PST) Rob said …
I’m not certain about different types of injectors. Perhaps there are constant current injectors today as you say. Then I agree it makes sense the opening time will be constant, regardless of battery voltage. I do know that earlier fuel injection systems simply applied battery voltage to the injector for the duration of the pulse. On these types of systems, the opening time is related to the battery voltage. The change in opening time was significant enough that fuel injection computers were (and maybe still are) programmed to compensate for it by sensing battery voltage and increasing the pulse width by an appropriate amount when the battery voltage was low. Using pulse width alone as an indicator of fuel consumption on such a system will yield a high estimate when the battery voltage is low. Based on the voltage profile I measured on my injectors, I see no indication that they are current controlled.

At 09:59, 6 March 2009 (PST) Daddyo said …
I initially thought you’d figured out how much energy/gas would be used starting a car vs idling and when it’s worth shutting your car off, including energy from the starter/battery which eventually makes its way from burning gas. I suppose you could use an ammeter on the starter cable then back off the efficiency of the motor-alternator-battery to figure out gas used to start might be a way.

At 13:13, 30 March 2009 (PDT) Rob said …
Hi Daddyo, The amount of energy drawn from the battery for starting a warm, well tuned engine is easily replaced by the charging system within the first minute of running the engine. Thus battery recharging is taken into account by my analysis looking at the cumulative pulse width in the first minute after starting (the first plot) and extrapolating a linear trendline backwards in time. Some of the additional fuel consumed is due to recharging the battery, some (probably most) is due to higher RPM after startup.

At 18:20, 17 May 2009 (PDT) Ron said …
Guys, have you any idea of the wear on an engine not set up for constant shut off and restart? Your oil pressure drops to zero. You have excessive cycles of low oil pressure operation, not to mention that your transmission is not designed to be brought into gear as you apply power all in the first second after starting. I appreciate what you are trying to accomplish, but don’t do it with a conventional engine. Buy a hybrid. They are “engineered” (to a degree) for this kind of “abuse.” Of course, that does not mean they don’t get beat up, but the car manufacturers see no downside to our replacing components rather then buying more gasoline. A lot of pollution is generated building transmissions, engines, starters and the like.

01:07, 18 May 2009 (PDT) Rob said …
Hi Ron. The fuel savings vs equipment lifetime debate is an old one. I purposefully did not address this issue because it tends to be hotly debated. Instead, I mentioned it as a disclaimer (#3) at the end of the article. While your arguments are logical, your conclusion, “don’t do it with a conventional engine” is not. In any choice there are costs and benefits. The existence of the costs does not necessarily mean the net result is negative.

Let’s throw out some numbers. Suppose engine lifetime is decreased by around 20%, suppose an engine normally lasts 100,000km (pessimistic) and suppose the cost of a rebuild is $1500. Thus the “engine cost” of driving is about $1.50 per 100 km and the cost of additional wear due to starting/stopping is 20% of that or about $0.30 per 100 km. The fuel cost for my GEO metro is currently about $5.00 per 100 km. So if starting/stopping decreases fuel consumption by 10% I will save $0.50 per 100km on fuel while paying an additional $0.30 per 100 km on engine wear, so I still come out ahead by $0.20 per 100 km.

I have no hard data to base those numbers on but I hope I’ve illustrated that even if there is significant additional wear, the net savings may still be positive. Also, the additional wear may be less than you think. There are many experienced drivers who have long been practicing hypermiling techniques such as stopping and restarting their engines rather than idling as well as pulse and glide and engine off coasting techniques. Many (myself included) have noticed little decrease in engine lifetime. I do not know for certain the reason for this, but at the risk of inviting further argument I will propose one:

While significant wear may occur during cranking of a cold engine since all the oil has drained into the crankcase, much less wear may occur during cranking of a warm engine that has only been stopped for a short period since there is still a significant amount of oil coating all bearing surfaces.

As for the transmission, it does not experience anything different from ordinary driving. It’s quite easy to start a warm engine, bring it up to speed, disengage the clutch and start driving within a couple seconds. It makes no difference to the transmission what the engine was doing prior to disengaging the clutch.

Starter lifetime will likely be decreased, but the cost of early replacement of the starter is still likely covered by the fuel savings.
Looking at GHG emissions or energy consumption as a metric instead of cost, the results are still likely to favor reduced fuel consumption over prolonged equipment life.

At 22:04, 2 June 2009 (PDT) B said …
Inspiring… Gonna start switching off my van more frequently at stoplights; before, I did it only when I could count on 30+ seconds stopped, but if the van dies of something, it aint gonna be starter motor failure, and it’d definitely save me gas just about every light I come to