Automatic Engine Start/Stop

Automatic engine start/stop systems start and stop the main diesel engine in an unattended fashion to provide a variety of features without requiring the truck’s engine to idle continuously. These systems have a set of inputs to ensure that it is safe to start the engine without anyone at the controls.

They provide some key features:

  • Heating and/or cooling the sleeper bunk while drivers are on a rest period
  • Maintaining adequate battery charge so the engine will start easily when the rest period is over
  • Maintaining engine coolant temperature so the engine will start easily when the rest period is over

There are two different types of automatic engine start/stop systems. The first has a primary 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 second type of automatic engine start/stop system focuses solely on maintaining the batteries’ state of charge.



They do not require additional HVAC components, batteries, or engines to accomplish their tasks

It is becoming increasingly common to pair an engine start/stop system with a battery HVAC system. This allows the engine to start automatically without driver input when necessary to keep the batteries charged and keep the battery HVAC system from shutting off if it runs low on available power

If a vehicle is purchased with a CARB 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


Noise and vibration of the system starting and stopping can disturb drivers. Drivers did not like the initial automatic engine start/stop systems that came out about two decades ago because when those systems started and stopped the main engine the noise and vibration was enough to wake a sleeping driver. Newer systems have made improvements, 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 engine shut down. If there is a large temperature difference between the desired bunk temperature and the outside temperature, the engine starts can become fairly frequent, exacerbating the sleep interruption problem. Another concern is the alarms that may go off as part of the engine starting process. The system manufacturers offer features to suppress these alarms when the start/stop system is enabled

Creates additional hours of wear on the main engine

It is not completely clear how various idle laws relate to some of the operational modes available with these systems. Refer to the resources listed in section 5.6 for the latest information on idling regulations

Fleet Survey Results

Automatic engine start/stop systems are one of the available anchor technologies of an overall idle-reduction strategy.

  • If a fleet does not use APUs, their idle percent tends to be in the 30% to 50% range
  • If a fleet uses APUs, idle percent is in the single digits to 20%
  • Fleets seem to be split on diesel APUs vs. battery HVAC systems with some fleets using both (but not on a single vehicle)
  • Use of auto start-stop systems is increasing
  • Almost everyone uses diesel fired heaters
  • Most fleets are looking for positive ROI, but most of them also think there is significant value in driver retention and hiring

Decision-Making Tools

Our team produced top-line tools to assist fleets in making their idle-reduction technology choice:

  • The confidence matrix summarizes the study findings and indicates NACFE’s confidence in the various idle-reduction technology solutions.
  • The Payback Calculator allows fleets to estimate the payback in months for various idle-technology solutions. Fleets input data into the form and the calculator uses the data along with the information gathered by the study team to quantify and benefits and consequences likely to be experienced by the fleet in terms of upfront costs and year-over-year costs.

In addition the report contains challenges and changes charts for each of the anchor technologies. The charts can be found in section 9 of the report.


Truck manufacturers provide numerous options when it comes to idle reduction and there are many suppliers of idle-reduction technology. No single truck manufacturer is capable of engineering, testing, sourcing, producing, and servicing the full range of available idle-reduction options.

Only one truck manufacturer currently offers an automatic start/stop system, although they are available from multiple suppliers.


As a result of research and interviews the study team reached the following conclusions concerning idle reduction in the sleeper cab market:

  1. Reducing idle is good for the environment, saves fuel and improves a fleet’s “green image.
  2. A 10% reduction in annualized idle percentage is worth about 1% in fuel economy that results in a $500 to $700 annual savings assuming 100,000 miles a year and diesel at $3 a gallon.
  3. Driver hiring, comfort and retention are mentioned in almost every conversation about idle-reduction technologies. Some have said that offering a good solution is a requirement for hiring and retaining drivers.
  4. There is no “one size fits all” solution to idle reduction.
  5. Drivers are still a very important part (If not the most important part) of successful management of idle times.
  6. Depending on “shore power” (outside electrical connection) to be available is not a reasonable idle-reduction solution for most fleets as the number of electrified parking spaces is extremely small compared to the number of trucks that need to be parked at any given time.
  7. Most suppliers of idle-reduction technologies mention that their solution helps minimize maintenance costs, particularly in the vehicle’s aftertreatment system. However, we have been unable to uncover any data that actually substantiates those claims.
  8. The answer to the question of the resale value of trucks equipped with idle-reduction technology is unclear.
  9. Maintenance of the vehicle’s battery connections and electrical systems is critical to getting the best performance out of idle-reduction technologies.