Stop/start explained

Is it fuel-saving technology or a means to fudge the numbers?

With fuel prices at an all-time high, automakers know that by quoting ultra-low consumption and emission figures, a purchasing decision may swing in their favour. By employing stop/start technology, manufacturers can lower their claimed fuel-consumption figures, but does this technology aid in achieving real-world savings? We take a look at the complete system and investigate its relevance to the consumer.


The idea behind a stop/start system is to reduce fuel consumption by shutting down (stopping) an internal-combustion engine when a vehicle is stationary. In the past, it was not effective due to less-than-perfect fuelling (think carburettors) and combustion during start up in a largely petrol-dominated passenger-vehicle market. The latest in injection technology (petrol and diesel) ensures that very little fuel is wasted during the starting event and makes stop/start systems a viable fuel-saving option … in theory.

Current emissions legislation forces manufacturers to design their cars to emit fewer pollutants across all their product offerings. This includes CO2, which is directly linked to fuel consumption. The claimed figures published for CO2 emissions and fuel consumption in Europe are linked to the New European Drive Cycle (NEDC). This cycle consists of urban and extra-urban driving, including a large percentage of idle time. Manufacturers have cottoned on to this fact and are currently decreasing their claimed figures by more than five per cent by employing stop/start technology. This is also the reason why this feature is filtering down to A- and B-segment vehicles.


The parts needed to employ this technology aren’t vastly different to those contained in a vehicle without stop/start. This might fool the ignorant car buyer into believing that they can recreate the benefit in their own vehicles by just keying off and starting their vehicles manually. By investigating the components of the system, it becomes clear why this is not a good idea.


Due to all the starting events, the battery will have to cope with frequent charging and discharging cycles. A standard lead-acid battery will overheat, which could dramatically shorten its service life. Battery manufacturers are now producing special stop/start batteries that are an evolution of the lead-acid battery to cope with the extra power and thermal requirements.


The design life of a standard unit assumes one starting event per drive cycle for the life of the vehicle. As the stop/start event may occur several times a kilometre in city traffic, a standard starter would not cope. Denso has gone one step further than developing a more robust starter by supplying a stop/start unit that can seamlessly engage into a moving engine to decrease starting times (see Techtalk, March 2012).


The alternator will have a harder task in ensuring a fully charged battery due to the frequent power-sapping start events. Stop/start alternators are specifically designed to cope with the extra power requirements. Some automakers employ regenerative charging, in which the alternator charges at the maximum rate during vehicle braking to increase the engine-braking effect and recover some of the kinetic energy. These systems are also termed micro or mild hybrids. Another possibility is to combine the starter and alternator in one unit called an integrated starter generator (ISG), which lowers component count and complexity.


The following sensors are necessary to employ the system:
• Wheel-speed sensor to determine that the vehicle is stationary;
• High-precision crank sensor to determine which cylinder is ready to fire first after a stop event in order to improve starting times;
• Neutral gear and clutch sensor to know when
it is safe to stop and start the engine in manual vehicles;
• Brake sensor to identify the brake-pedal status;
• Accelerator position to determine the driver demand.


Stop/start software that contains all the control algorithms and  ow diagrams resides in the ECU of the vehicle. It is the ECU’s task to control the automatic stopping and starting of the engine in a safe and unobtrusive way. Please read the owner’s manual of your vehicle to know exactly which conditions will allow the stop/start system to function. A vehicle fitted with a manual gearbox will usually require the transmission to be in neutral, the clutch pedal up and the driver’s foot on the brake pedal to stop the engine when stationary. In the case of an automatic transmission, a simple foot on the brake when the vehicle is stationary does the trick. A display in the instrument cluster will alert the driver that the stop/start system is active to avoid manual starting or the possibility of the driver getting out of the vehicle. Any driver action detected by the ECU that suggests a requirement to move the vehicle will result in the automatic restart of the engine.

Generally, the engine will not shut down if the battery’s charge is too low or the climate control set-point deviates too far from the interior temperature (mostly in vehicles where air-con compressors rely on engine power to function). The driver can usually override the system with the press of a button.


Occupants of a vehicle expect the same level of creature comforts in the cabin when the engine is stopped. This is not possible with belt-driven ancillaries such as air-con compressors. The new trend is to power these with electric motors, which ensure full cabin functionality during engine-shutdown periods. Eventually, the influx of hybrids and EVs will render this technology obsolete, as driving on battery power alone on congested roads makes the most environmental and financial sense.


Is a stop/start system worth the effort? If you are stationary for fewer than five minutes during your commute, the system will generally not result in significant fuel savings. However, it might be a good idea to live with the irritation of extra vibration as well as a slight delay in engine response from rest. In the end, every drop counts.


WE decided to create our own severe city-drive cycle and tried to replicate the exact drive (including stops, vehicle speed and distance covered) with the stop/start system active and disabled. Although not completely scientific, in theory this experiment should have shown the benefits of the technology.


Total Distance (km)                                             6
Number of one-minute stops 10
Number of five-minute stops 4
Total time stationary (minutes) 30
Total time to complete test route (minutes) 47
Maximum speed during test route (km/h) 40
Average speed during test route (km/h) 8


The test car was a Mercedes-Benz B180 CDI AT (see road test on page 70) with all ancillaries (example, air-con) switched off for the entire test. During the run with stop/start active, the system failed to switch off the engine after 11 stops, which meant that the engine was off for a total of only 12 minutes and not the 30 minutes originally planned. This can possibly be attributed to a low state of charge of the battery due to limited engine run-time between the stop/start procedures.


Fuel consumption without stop/start (L/100 km) 11,3
Fuel consumption with stop/start (L/100 km) 0,3
Total fuel savings per 100 km (L) 1,0
Money saved per 100 km with diesel @ R11,49/L (R) R11,49
Total fuel saved during 6 km test cycle (L) 0,06
Money saved during the 6 km test cycle with diesel @ R11,49/L (R) R0,69


A modern 1,8-litre diesel engine such as the one in the B-Class will use an estimated 0,6 L/h during idle when the engine is at operational temperature (with ancillaries switched off). Therefore, an ideal stop/start system should have saved 0,3 litres of fuel during the 30 minutes of stationary time (an estimated saving of R3,45 at our diesel price). Unfortunately, this figure could not be realised in real-world testing because the system allowed the engine to switch off for only 12 minutes and therefore decreased the saving to 69 cents.

Article written by

CAR’s technical editor was once the senior vehicle integration engineer on Optimal Energy’s Joule electric car and had stints with Ford (UK), Integral Powertrain (UK) and CAE. He completed his MSc in mechanical engineering in 2004, obtained UK Chartered Engineering status in 2009 and is a judge of the International Engine of the Year competition.