Direct injection

Will the starter motor soon be out of a job?

Direct-injection petrol engines can be started directly without a starter if the control system is suitably sophisticated. New systems are about to go into series production.

Direct injection petrol engine: direct start relieves the starter motor

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Only a few years ago, people were still arguing about how long it would take to switch off the car engine at traffic lights for reasons of consumption and emissions. Depending on the ideological message that wanted to be conveyed, the figures ranged between about 10 and 30 seconds.
Today, when it is well known that everything has to go a little faster in all areas, the aforementioned switch-off interval is also assessed quite differently. If the start-stop system is optimally tuned, a positive effect on fuel consumption is said to occur from an engine standstill time of around 1 second.

Start-stop systems
Today, more and more vehicles are being equipped with so-called start-stop systems, as these can further reduce fuel consumption values - depending on the operating conditions to a marginal or significant extent.
Thanks to the continuous development of system components, more and more vehicles and drive systems are able to integrate such systems. Petrol and diesel engines can now be restarted quickly and reliably; even vehicles with torque converters are capable of this (see AR 18/2008). There is also potential for optimisation in this area with the so-called direct start: starting the engine directly means that neither a starter motor nor a crank handle and muscle power are needed. Accordingly, a direct start system can save both weight and installation space.
An indispensable prerequisite for the functioning of the system is an externally ignited, jet-guided direct-injection combustion engine. Many of these engines have come onto the market in the recent past, so that the time for the series introduction of the first direct-start system is approaching.
The petrol engine can be started without a starter if fuel is injected into a combustion chamber with pistons at top dead centre, followed by an ignition pulse. However, because towards the end of this combustion process another piston is at the end of the compression stroke and thus generates a large counterpressure, this single ignition is not sufficient to start the engine safely.
For this reason, the tuning of piston position, valve train, injection and ignition timing is extremely difficult. The engine must be able to be stopped in a precisely defined position for restarting. In addition, the thermal conditions in the engine to be started must be taken into account. If the engine is too cold, the starting torque is not sufficient for a safe start; if the temperature is too high, the air density is too low and the cylinders are not sufficiently filled.

Supporting starter
With the current state of the art and in view of the considerable difficulties in the practical realisation of such systems, the developers reckon that although direct starting is possible, a starter is still necessary for reliable functioning under all conceivable operating conditions. This starter could, however, be significantly smaller in size, thus contributing less weight and requiring less energy to operate. In this case, the developers speak of "starter-assisted direct starting". It is not yet known who will finally bring the first vehicle with a direct-start petrol engine onto the market, but German and Japanese suppliers are likely to be at the forefront. Mazda, for example, will equip some models with the SISS (Smart Idle Stop System) direct-start system from 2009.
So it won't be possible to do without the good old starter motor any time soon. However, we will increasingly see it in a much slimmer form.
You can find the article in issue 19/2008 of "Automobil Revue", which you can of course also subscribe to online.

AR 19 of 7.5.2008