Intro

Recently I heard that starting your car engine consumes less fuel than 10 seconds of idling. Being the curious type, I looked for further information and found opinions varying anywhere from 5 seconds up to 1 minute. Therefore I decided to experiment on my own car to determine the truth.

Procedure

1. Connect a line from the audio line-in of a laptop to your fuel injector to measure the fuel injector pulses. The procedure is described here: http://www.gassavers.org/showthread.php?t=4250&page=2. You do not need to connect to the VSS sensor since we’re only interested in the fuel injector for this experiment. Note that my car has only one injector. Therefore I know that by probing that injector alone I’m effectively measuring all the fuel going into the engine. Fuel consumption is proportional to fuel injector pulse width. If your vehicle has multiple injectors, you cannot know for certain that you are measuring all the fuel unless you probe every injector, but likely probing a single injector would provide reasonable results.

2. Bring the engine to normal operating temperature. Then turn it off for 10 seconds or so, start recording a wave file on the laptop, and turn the engine on (without depressing the gas pedal) and let it idle for a minute or so before you stop recording.

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3. Write a nifty program to parse the wave file and record the start and end times of each pulse. I wrote the wave file parser in Excel with VB for applications. If you want to repeat the experiment, you’ll likely need to make some modifications to the VB code to get it to work with your engine (more details in the Excel file).

Idling vs starting.zip – This zip file contains the wave file as well as the Excel file I created to parse the wave file and calculate and plot the results shown below.

Results

This shows the cumulative injector pulse width, effectively the total fuel consumed. You can see that initially fuel is consumed at a faster rate (the slope of the curve is greater) but eventually if falls off and becomes linear indicating a constant rate of fuel consumption at standard idle speed. The black line is a linear trend line based on the last 5 seconds of data (from 55 to 60 seconds). Extrapolating this line backwards we can see how much earlier we would have had to start the clock in order to consume the same amount of fuel from just idling. The time is about 7 seconds before 0. This means that starting the engine (followed by idling) consumes the same amount of fuel as 7 seconds of idling (followed by more idling).Note that this value accounts for any additional fuel required to recharge the battery to it’s pre-crank state. A well tuned, warm engine starts very easily and 1 minute of idling is plenty to recharge the battery. Further testing would be required to determine how much of the 7 second value above results from the higher idle RPM and how much results from recharging the battery. Given how closely the injector duty cycle curve below matches the RPM curve, it seems likely that most of the additional fuel is due to higher RPM and that battery recharging has a negligible effect. A good additional test would be to measure battery power draw (voltage * current) during regular idling and just after starting for comparison.

This is a closeup of the cumulative injector pulse width for the first 3 seconds. The dark blue points are the actual injector data. It’s interesting to note that although the injector pulse width is much greater for the first second or so (see pulse width plot below) the fuel consumption rate (as indicated by the slope of the cumulative pulse width curve) is actually lower since the engine is at such a low RPM. I’ve heard many claims that the fuel consumption rate at startup is much higher than normal but that does not appear to be the case.The three straight lines show what the consumption would be at standard idle speed for comparison. The 7 second equivalent idle time shown in the above plot is misleading because it assumes the engine is just left idling after starting in which case the additional fuel consumed is mostly due to a higher idle speed setting for the first 20 seconds or so (see plots below). But regardless of whether you were idling or stopped, you will likely press the gas pedal within a few seconds after t = 0, and then your fuel consumption will be the same in either case. The three lines shown are all the same slope as the linear trend line in the above plot showing normal idle speed fuel consumption. They have been offset to intersect the fuel consumption data curve at 1, 2, and 3 seconds respectively. It is apparent that the “idle time equivalent of starting” is related to how quickly you start extracting useful work from the engine after starting it. If you can do this within 1 second (which is not difficult), then restarting the engine consumes the same fuel as idling for about 0.2 seconds. If you take 2 seconds, then restarting the engine consumes the same fuel as 0.7 seconds of idling. If you take 3 seconds then restarting the engine consumes the same fuel as 1.3 seconds of idling. Unlike the 7 second value in the first chart, these values do not take into account fuel that may be consumed to recharge the battery to its pre-crank level. Therefore the true value may be a little higher but will still likely be much less than 7 seconds.

This shows injector pulse width as a function of time. Injector pulse width is an indication of fuel consumption per revolution. Note that the injector pulse width is much greater for the first second or so. Effectively the engine is doing more work per revolution during this period in order to accelerate up to idle speed. The injector pulse width is also an indicator of energy input per revolution. Energy is equal to force times distance. The distance in this case is constant (one revolution). Therefore, the injector pulse width is also an indicator of force (or more appropriately torque since we’re talking about rotation). The torque is high when the engine is accelerating quickly up to idle speed, but drops to a roughly constant value when the RPMs are not changing quickly. This constant pulse width at idle can be thought of as representing the torque required to overcome engine friction. The pulse width must be greater than this in order for the engine to do any useful work.In spite of the higher RPM for the first 20 seconds or so, the pulse width is relatively constant. This indicates that the torque required to overcome engine friction is roughly constant, independent of RPM.

This shows the injector pulses per minute as a function of time. The injector is fired twice per engine revolution. Therefore the plot should be exactly twice the RPM. Looks about right.

Whereas the injector pulse width indicates the amount of fuel injected per revolution, the duty cycle indicates the amount of fuel injected per unit time, or in other words, the fuel consumption rate. The fuel consumption rate in litres per hour is equal to C * duty_cycle where C is a constant that could be determined by further experimentation. Notice that the fuel consumption rate does not show any significant spike near the beginning as one might have expected. The curve matches the RPM curve almost exactly, except during the first second of acceleration when fuel is consumed faster to bring the engine up to speed. Effectively during the first 500ms, enough energy (fuel) must be added to overcome engine friction and store some kinetic energy in the engine in the form of increased RPM. After the first second, this curve matches the RPM curve almost exactly, indicating the fuel is only overcoming engine friction and not doing any additional work. In other words, after one second, the increased fuel consumption for the remaining 20 seconds or so is due only to the higher RPM setting that kicks in just after starting.

Conclusion

It appears to be true that starting your car consumes less fuel than 10 seconds of idling. In my case it consumes about the same amount of fuel as 7 seconds of idling. However, the additional fuel consumption observed seems almost entirely due to a faster idle speed setting for the first 20 seconds after starting. Any good driver would start moving within 1-2 seconds after starting, which would effectively eliminate the fast idle losses by extracting useful work from the additional fuel being injected. If you can begin extracting useful work from your engine within 1 second after starting the engine then it appears starting the engine consumes fuel equivalent to about 0.2 seconds of idling. This is not accounting battery re-charging after starting, but that appears to have a negligible effect.

How much fuel can be saved? This will depend heavily on the environment in which you drive and the degree to which you are willing to turn you engine off. Using engine-off-coasting techniques described in How to reduce your vehicle’s fuel consumption I’ve found that in a typical commute through the city it’s not difficult to have the engine off about 30% of the time (without adding any time to the trip). Let’s assume a typical city commuter drives 15km in 30 minutes each way to work, 250 days a year (250 hours and 7500km in total just commuting to work, not including any other driving). A ’92 Geo Metro consumes roughly 0.5 litres per hour at standard idle speed and consumes about 5 litres of fuel per hundred km of single occupancy travel. Therefore, if I were a typical commuter, my savings would be about 75 hours of idling or 37.5 litres per year out of about 375 litres total or about 10%. If you only turn your engine off when the vehicle is stopped (ie you don’t use engine-off-coasting) you might expect to see only a 5% increase in mileage.

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Disclaimers

The above experiment is for a ’92 Geo metro. Results may vary from one vehicle to another. This experiment was performed on a warm engine. The results will be different on a cold engine since the engine management system injects additional fuel during a cold start. However, the standard idle speed is also higher on a cold engine, so the effects may cancel. Further experimentation would be required to determine how many seconds of cold idling is equivalent to a cold start. Generally it’s been recommended to me not to stop and start a cold engine, but I’m not certain if the reasons for that are based on actual data or just opinion. I have little faith in popular opinion without data to back it up. I have only considered fuel consumption and not overall cost or overall impact (GHG emissions, pollution, etc). Stopping and starting your engine frequently may cause faster wear on the engine and starter and may result in additional costs for early repair/replacement of components and additional energy consumed to repair/replace those components. However, I question whether those costs would outweigh the savings of a 10% reduction in fuel consumption.