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  • Jeff Lynch

Pros and Cons of Automatic Engine Start/Stop Solutions

The idea of ‘Idle Reduction’ is broad, complicated, and includes many technologies and solutions that might work well together or conflict with each other. Each solution promises some type of benefit, but there is never a silver bullet that can be a perfect idle reduction solution.

The North American Council for Freight Efficiency (NACFE) put together an excellent review and report of all the idle reduction solutions on the market today listing the advantages and disadvantages of each solution from a neutral position and looking at just the facts and research. If you aren’t familiar with NACFE, it is a great organization that provides in-depth and quality research to help fleets run more efficiently.

The following is a copy of the NACFE’s report on automatic engine start/stop solutions. You can read the full NACFE Confidence Report by clicking here.

Overview of Automatic Engine Start/Stop Solutions


Once the engine is running the vehicle’s HVAC systems will warm or cool the sleeper just as they would when the truck is driving down the road. Essentially the vehicle is still idling, keeping the engine warm and the batteries charged, but it is able to do so much more intelligently/efficiently, and automatically.

These systems can be used in very cold climates, for instance while a truck is in storage for a weekend or other down time, and offer very beneficial results, ensuring the vehicle will start when it is time to go back into freight-hauling operation. Automatic engine start/stop systems perform the work of both a block heater and battery charger without the need for the truck to be connected to outside power.

Types of Automatic Engine Start/Stop Systems

There are two different types of automatic engine start/stop systems, the most common being one that has a goal of maintaining a cab’s interior temperature when the vehicle is occupied. These may also assist with keeping the engine warm and the batteries charged.

The newer and less common type of automatic engine start/stop system focuses solely on maintaining the batteries’ state of charge. Given the growth in use of battery HVACs, combined with the new HOS restarts that last far longer than the 8 to 12 hours a battery HVAC can operate on one charge, this type of automatic engine start/stop system will probably grow in popularity, as they serve to recharge the battery HVAC system as it has drawn itself down. These will therefore enable battery HVACs to idle for longer than a single 8-10 hour window, allowing a driver to enjoy air conditioning continuously during an HOS restart. This type of automatic engine start/stop system is programmed to recognize the specific type of batteries being used by the battery HVAC system, and monitor their voltage, current draw, and temperature to provide optimal recharging patterns by comparing the inputs they receive to electronically stored battery-life models. It will typically require about 45 minutes of engine operation (which the automatic engine start/stop system will control) to fully recharge a battery HVAC system for an additional 8-10 hours of operation.

Advantages of Automatic Engine Start/Stop Systems

The most obvious benefit of all automatic start/stop systems is that they add few components and little weight to the vehicle. Since they are controlling the main engine, they do not require additional HVAC components, batteries, or engines to accomplish their tasks.

If a vehicle is purchased with a California Air Resource Board (ARB) “Clean Idle” engine, it will have a serial numbered holographic sticker on the driver’s side of the hood or driver’s door. Such stickers allow automatic engine start/stop systems to be used without violating any idling regulations, provided that the vehicle wasn’t also purchased with the tamperproof five-minute timer which does not allow any idling at all beyond that time limit.

Clean idle engines offer an integrated and clean solution which utilizes a few extra sensors to provide all of the desired HVAC and hotel load benefits while the vehicle is not moving. It may also be the solution most preferred by fleet maintenance teams, given its simplicity/commonality of diagnostics, service and parts.

Disadvantages of Automatic Engine Start/Stop Systems

The initial automatic engine start/stop systems that came out about two decades ago were not well-liked by truck drivers. Whenever those systems started and stopped the main engine the noise and vibration could wake a sleeping driver. Some improvements have minimized this problem on newer systems, such as using the engine brake to create a more rapid and smooth engine shut-off than the cab rocking and engine sputtering which occurs during a normal shutoff.

Another drawback of these systems is that they do require idling the main engine, creating additional hours of wear on the main engine and loading of the DPF exhaust system.

Finally, it is not completely clear how various idle laws relate to some of the operational modes available with these systems. If the automatic engine start/stop system is charging the batteries or allowing a regeneration of the DPF system, it should be permissible to allow engine operation longer than the typical five minute maximum, similar to the rules which govern aspects of the electronic engine idle parameters technology class, and are detailed in that section.

Recommendations and Best Practices

If your fleet is dissatisfied with the length of operation of your battery HVAC system, a battery monitoring and charging system such as that offered by automatic engine start/stop systems could be a desirable investment to extend operation.

For fleets that are challenged from a support aspect to keep diesel APUs systems in operation, the combination of a Clean Idle engine and an automated engine start/stop system could reduce that challenge.

Again, the information in this post is from NACFE’s report on automatic engine start/stop solutions. You can read the full NACFE Confidence Report by clicking here. Feel free to visit our home page or contact us directly at 913-744-4353 to see if Idle Smart can help you.

Interested in reducing the idle time of your fleet? Check out Idle Smart's Services to see if we're a good fit for your fleet.



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