Managing ignition timing

By Marcus Wilson 13/06/2014 5


I've just been at a great lecture by Peter Leijen as part of our schools-focused Osborne Physics and Engineering Day.   He's an ex-student of ours, who did electronic engineering here at Waikato – and graduated just a couple of years ago.  He now works in the automotive electronics industry. That's an incredibly quickly growing industry. So much of a car's systems are now driven by electronics, not mechanics. Being a car 'mechanic' now means being a car 'electronic engineer' just as much as it does being a mechanic. 

One interesting piece of electronics is the ignition timing system. The mechanism that produces the spark in the cylinders from a 12 volt battery is really old and standard technology – one uses a step-up transformer and kills the current to the primary coil by opening a switch – the sudden drop in current creates a  sudden reduction in magnetic flux in the transformer, and these collapsing flux lines cutting the secondary coil create a huge voltage, enough for the spark plug to spark. That really is easy to do. The tricky thing is getting it to spark at the right time. 

One needs the fuel/air mix in the cylinder to be ignited at the optimum time, so that the resulting explosion drives the piston downwards. Ignite too early, while the compression is going on, and you'll simply stop the piston rather than increasing the speed of its motion. Apply too late, and you won't get the full benefit of the explosion. It's rather like pushing a child on a swing – to get the amplitude of the motion to build, you need to push at the optimum time – this is just after they've started swinging away from you. 

All this is complicated by the fact that the explosion isn't instantaneous. It takes a small amount of time to happen. That means, at very high revolution rates, one has to be careful as to exactly when you make the ignition. It has to be earlier than at lower rates, particularly if the throttle setting is low, because the explosion takes a significant proportion of the period of the oscillation.  This is called 'ignition advance'.  

On newer cars, this is done electronically. A computer simply 'looks up' the correct angle of advance for the rpm and the throttle setting of the car, and applies the outcome. The result: a well running, efficient engine, using all the power available to it. Or so you might think.

But here's the revelation from Peter: car manufacturer's can deliberately stuff up the timing. Why do they want to do that? Well, there's a market for selling different versions of the otherwise same car. The high-end models have performance and features (and price tag) that the low-end models don't have. There's status in buying the high-end model (if you're the kind of person who cares about that – and the fact that these things sell says, yes, there are such people), but, alternatively, if that extra couple of horsepower doesn't bother you, you can get the lower-spec model for a lower price. Now, the manufacturers have worked out that making lots of different versions of the otherwise same car is inefficient. It's far easier to have a production line that fires out identical cars. So how do you achieve the low-end to high-end specification spectrum? Easy. You build everything high-end, and then to produce  low-end cars deliberately disable or tinker with the features so they don't work or don't perform so well. That is, make the car worse. 

Ignition timing is one example, says Peter. There are in fact companies who will take your low-end car and un-stuff-up your electronics for you – in effect reprogramme it to do what it should be doing. In other words, turn your low-end car back into a high-end one (which is how it started out life) without you having to pay the premium that the manufacturer would place on it for not stuffing it up in the first place. 

Who said free market economics resulted in the best outcome for consumers?

 


5 Responses to “Managing ignition timing”

  • Back in the late 1970’s there was a project in the Physics Department at Victoria University to build an engine control system for greater fuel economy. It was supported by the New Zealand Motor Corporation (with the generous donation of an Escort van) and employed a number of post-graduate Physics and Engineering students on a part-time basis (nice work if you could get it).

    The point is: these microprocessor controlled engines were being developed by big automotive firms overseas for relatively expensive cars, but those projects aimed to enhance performance not fuel efficiency. Dr Jim Morris, who ran the project, saw an opportunity to optimise for fuel economy instead. (NZ had suffered from oil shocks and fuel economy was an issue.)

    So, I think it is probably too simplistic to say that the different settings in modern engine control systems are there make the car perform less well. They may give better economy and longer engine life, perhaps?

    By the way, Jim’s project was running at the time that Apple came out with the Apple II (no IBM PCs yet) and Space Invaders was a major source of distraction in the lab! In those days, you could actually build your own microcomputer! It was a very interesting project.

    • Sorry for the late approval of this comment! Yes, I’m sure there’s more to it than just making the car perform less well, but that’s what Peter said!

  • Since the introduction of stringent emissions and fuel economy controls, new car engines have to be highly combustion efficient over their normal operational range (CE = 99.5+%), this is achieved by a mixture of engine design, emissions controls, and engine management. Simply modifying ignition timing would cause vehicles to fail emissions. With the recent move to controlling carbon dioxide emissions, there has also been a drive to improve thermal efficiency ( TE = 30 – 50%, at best), especially in petrol engines.

    Obviously I didn’t attend the lecture, however no major car engine manufacturer would deliberate hobble an engine in any vehicle model by modifying ignition timing.

    Today’s vehicles use a PCM ( power-train control module ) that incorporates the engine control unit (aka ECU/ECM) and transmission control units, with the major goal of minimising regulated emission and maximising combustion and fuel efficiency of the specific engine/vehicle combination.

    The PCM has inputs from many sources, including sensors such as mass air flow, air intake temperature, engine speed, transmission gear, fluid temperatures from engine and transmission, engine crankshaft angle, throttle position, camshaft angle, engine knock, and various emissions control sensors – such as oxygen.

    A PCM can adjust fuel pump and injectors ( carburettors are long gone ), engine ignition timing, adjustable camshaft position, engine speed, engine cooling fan, emission system optimisation ( cold starts usually require more fuel, forced air induction controls (exhaust turbo, superchargers ), crankshaft position, transmission gear selections, and contribute to traction control systems.

    The system uses knock sensors to adjust the engine operation map for the current fuel and vehicle load. When a lower octane fuel, or a fuel containing oxygen ( because of alcohol additives ), the engine changes many settings to maintain maximum combustion efficiency at the required engine power/torque output.

    The ignition timing will be changed but, much more importantly, so will the fuel system parameters, along with other parameters ( engine valve timing, forced air induction, transmission gear change points, emission system settings, etc. etc. ).

    For modern, lighter vehicles, one occupant cruising at 100 km/h on the flat has quite different engine/transmission operation parameters to when it has 3 passengers and a loaded trailer going up Ngauranga Gorge. The PCM adjusts parameters for optimum emissions, performance, and economy – depending on predetermined settings ( engine mapping ) and/or selected drivetrain mode ( performance, economy etc. ).

    However, in all cases, ensuring compliance with emissions regulation means that many parameters will be adjusted, of which the fuel system is the most important, depending on fuel and emissions.

    A modern engine will be used over several model ranges, and can be used with a range of fuels, however the vehicle manufacturer will have spent serious money ensuring the engine map has been modified and optimised for the target vehicle, and also meets regulated emissions requirements, something not possible by merely changing engine ignition timing.

    Sure, there are aftermarket ECUs available, which can modify settings to increase output power/torque profiles, usually at the expense of combustion efficiency ( thus more regulated emissions ). Aftermarket ECUs are normally illegal for road use in countries like the USA, unless certified for the vehicle.

    NZ has lagged behind other 3rd world countries with regard to reducing vehicle emissions, but hopefully the recent and ongoing Japanese trend to small, lighter, and fuel efficient vehicles will eventually arrive here as we continue to purchase their cast-offs.