Engine Management Systems – An Explanation

Engine Management Systems – the way different systems work. (Copyright Torque Developments)

I regularly get asked questions about the different methods of load sensing used by different engine management systems, people often ask me why there are even different methods at all, and perhaps the most common questions I get asked are about which method is best suited to a specific application. Well in keeping with the ethos behind this technical forum the following information should prove helpful for those curious to understand a little bit more about the ‘how and why’ of the engine management world, I’ll attempt to briefly explain the methods behind most of the common strategies and then I’ll bullet point some of the pro’s and con’s for each type of system.
Before I get into the details let’s just briefly re-cap some of the basics about engine management systems, firstly let’s just state that the primary job of the engine control system is to provide our engine with sparks at the right time and fuel in the right quantity but also at the correct time. Engine management systems live life at a frantically fast pace, it lives life from just one engine cycle to the next, its entire world is the current engine cycle.
• The job that engine management systems must complete for each engine cycle is to first measure all of the controlling variables which control its internal look-up tables (or maps) as it is required to make a firm decision quickly on the ignition event angle which is to be used for this cycle, then it must also make a firm decision on fuel delivery and decide on an injector opening duration.
• Once our ECU has weighed up the variables and has settled on both an ignition event angle and an injector opening time it now has to begin to charge our ignition coil in preparation for the ignition event, also it must decide when in our engines cycle to begin the opening of the fuel injectors.
• Whilst injecting the estimated required amount of fuel the ECU waits for exactly right time to cease charging the ignition coil.
• At the critical point our ECU cuts the electrical feed to the ignition coil causing the nicely contained and highly charged electrical field which has built up inside the coil to collapse and desperately search for an escape route.
• This newly homeless energy tries to tunnel its way out of the coil the easiest way it can which should be down through our spark plug, the energy courses down through the spark plugs core eventually jumping through our compressed charge in the combustion chamber and over to the spark plug’s earthing strap, by doing this we start combustion process for this engine cycle.
At 6000rpm the ECU has just 20 milliseconds maximum to complete this entire process, a good quality modern ecu will do all of this and more with plenty of time to spare.
MAF – Mass Air Flow

I’ll start here as these engine management systems are far and away the most popular in the world, at least in terms of numbers, this method is used by 90% of the worlds OEM vehicle manufacturers. This system will employ some type of sensor to take direct real-time measurements of the air mass consumption of the engine. Some of the early MAF sensors were not much more than a barn door with a variable resistor attached and held closed by a soft spring, airflow progressively pushed the door open so the sensor reacted electrically to increased airflow. More modern MAF sensors disrupt the airflow less and use hot wires or hot films to try to gauge the air mass movement over the sensor. One common feature across all MAF sensors is that they must interfere to some extent with the engines intake airflow and so can hinder efficiency.
How does it work?– This system estimates our engines volumetric efficiency by knowing in one hand the engines physical capacity and in the other hand it knows how much air mass it’s just eaten. It is possible from this information to estimate the required fuel delivery and to less certain extent the required ignition advance for any given engine cycle.
Why MAF? – Well if you’re a high volume car manufacturing company then it’s a very logical choice. The big problem for major OEM’s is that it is logistically impossible to build accurate engine management systems calibration of every single individual engine that rolls off of the production line. It goes without saying I’m sure that not all production engines are equal, manufacturing tolerances will always give rise to some differences from the best engines to the worst and therefore engine management systems must be able to “see” the differences between these engines and effectively roll with the punches. So engine management systems which constantly measures the engine’s efficiency are a handy tool, albeit not perhaps the ideal solution from an engineering point of view. Pro’s –
• Robust and easy to calibrate in a way that allows one ECU calibration to be copied across many vehicles. • Copes well with dynamic changes in the engine’s efficiency throughout the vehicle’s life, for instance blocked exhaust systems, etc.  Con’s-
• The system must make the assumption that all of the measured air mass does in fact go through the engine and also that the engine can get air from nowhere else other than through the MAF sensor, this assumption does make these systems extremely vulnerable to leaks. • These systems are unable to assess the exact intake charge density where it really matters which is directly behind the intake valves, unfortunately it is this highly localised density that really does affect the combustion dynamics and so with it the required ignition event angle, the upshot is that our ignition control is not so good with a MAF system. • MAF sensors do tend to have a fairly limited shelf life and rarely last the entire operational life of a vehicle, as and when they do start to fail the results can vary from a fairly harmless reduction on output and increased fuel consumption right across the spectrum to a spectacular total engine failure.
Alpha-N

The most complex mysterious sounding name here actually describes the simplest of engine management systems, the Alpha part of the name refers to an “Alpha” variable, this variable can technically be any relevant variable but most commonly it’s the voltage reading from the throttle position sensor (TPS) which is used. And the N part of the name would normally refer the engine speed (RPM), I’m pretty sure there would be a lot less confusion in our world if it were to be referred to as just TPS vs RPM.
How does it work?– In this system the look-up tables (maps) are driven by engine speed along one axis and throttle plate position along the other. This system relies very heavily indeed on the engineer calibrating the system accurately as the system itself is blind to a lot of critical sub-variables such as the mass air flow and the intake manifold pressure, the chosen alpha variable tenuously controls these as long as everything else remains equal. The calibration engineer must visit every single area of the look up tables and set each to the ideal ignition timing values as well as the ideal fuel delivery values, this must be carried out on an engine by engine basis and the calibration will only ever be correct for as long as the engines volumetric efficiency profile remains exactly as it was on the day of the actual calibration, a consistent performance from the exhaust and intake systems is also blindly assumed.
Why Alpha-N? – Well, Alpha-N is simple for the ECU to manage, the burden is on the calibration engineer not on the ECU so the actual hardware is often quite basic in nature and therefore quite low cost. This method of load sensing is extremely flexible and can be quickly applied to almost any design of engine, it’s especially good at dealing with very hectic harmonic environments such as very short individual throttle body set-ups on smaller engines using very long duration camshafts.
Pro’s – • Straight forward to understand and calibrate • Low cost • Low parts count • Good at dealing with large amplitude intake system harmonics.
Con’s – • The system is vulnerable to post throttle air leaks • The system cannot self-adjust for dynamic changes in the engine volumetric efficiency • The system cannot cope with forced induction applications whilst only using tps vs rpm • Throttle position sensors have a relatively short shelf life and this system absolutely relies on having an accurate TPS signal at all times.
Speed Density 

These rae the engine management systems you will find controlling most motorsport engines especially those utilising forced induction. The name refers to the two major drive variables for the strategy engine SPEED and the intake charge DENSITY.
How does it work? –To know the density of a gas we need two essential pieces of information, pressure and temperature. The pressure would be monitored by a Manifold Absolute Pressure (MAP) sensor, the temperature of the intake charge is measured normally with a thermister which is nothing more than a resistor that changes resistance as it’s temperature changes.
It’s probably worth mentioning at this point that MAP in this instance is an acronym for Manifold Absolute Pressure, the absolute part refers to the fact that in the world of engineering and science it’s useful for us to talk about pressure in a slightly different way to the generally accepted norm.  In absolute terms zero pressure is what you might commonly call a total vacuum, and in absolute terms as you sit reading this you are most likely sitting in air which has a pressure of approximately 1000mbars.
Why speed density? – When accuracy of engine control is the most important factor then speed density is about as good as it gets, by accurately measuring intake charge density real time behind the intake valves we can cater for the real life, cycle by cycle combustion environment precisely. This system will look up the right ignition and fuel numbers regardless of any leaks whether pre or post throttle body so it is highly robust. Pro’s
• Highly relevant assessment of the intake charge conditions which results in highly relevant ignition event timing and fuel delivery look up. • Robust against common real world faults like intake system air leaks. • Highly adaptable to variable running conditions. • Very suitable for forced induction applications.
Con’s
• These systems still rely very heavily on the calibration engineer to program the tables accurately as the system cannot see VE. • Correct positioning, plumbing and mounting of MAP and intake air temp sensors is critical for accurate operation. • More time consuming to calibrate than other engine management types.
For ultra accurate engine control we can combine aspects of the various systems, for instance speed density systems can be combined with Alpha-N control so that layers of pressure driven 3d look-up tables can be cycled through in a fourth dimension by an Alpha variable such as TPS. This might be useful when controlling a turbocharged application with throttle plates mounted very close to the intake valves, as in this instance different throttle plate angle might well change the eventual combustion environment in the cylinder by altering the local aerodynamics in the intake port but without necessarily changing the measurable charge air density. This is perhaps beyond the scope of this first post but worth a mention for those who are up to speed on this subject.
Written by Sam Borgman 2011

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