We want the best of all worlds, and messages from government try to convince us this is deliverable. We want cars that are cheap, frugal and that don’t pollute the environment or trigger climate change, aircraft that allow us to holiday in the Bahamas or Thailand without building new runways or making life unbearable under their flight path and a health service that provides ever-increasing amounts of care to an ageing population without putting up taxes.

In reality, all these require compromises and the design of diesel engine control systems is no exception. We were encouraged to switch from petrol to more efficient diesel cars to reduce carbon dioxide emissions. One of the reasons diesel cars are more efficient is that they operate at a higher pressures and combustion temperatures. However, the higher temperatures encourage a greater fraction of the oxygen and nitrogen in the air to combine into oxides of nitrogen (NOx) which is a serious risk to health.

The designers of car engines are faced with a series of options to obtain the best (or possibly least bad) compromise between performance, flexibility, fuel efficiency, carbon dioxide (CO₂), carbon monoxide, nanoparticles, and NOx emissions. They have many possible tools at their disposal, including varying valve timing and the amount and timing of injected fuel, variable geometry turbochargers, the addition of chemicals, such as urea, into the exhaust to react with NOx, the choice of different catalytic converter materials and the recirculation of some of the exhaust gases.

The optimum solution is different, depending on the power demand, engine speed, air temperature, altitude, exhaust temperature and other factors. The decisions on how to obtain the best compromise are made in an electronic engine control unit (ECU).

The engineers setting the parameters for the ECU and validating them on the test bed will be aware of the importance of reducing CO₂ emissions to lie within certain tax bands and that NOx has to be kept below a certain level. They will also know the conditions under which the car will be tested to check compliance with these limits. Given the importance of meeting these standards, it is hardly surprising if they adjust the parameters in the ECU to ensure that, under conditions similar to those in the test cycles, the optimisation is tilted towards emissions reduction, rather than performance. This is not cheating, it is normal practice when trying to meet a range of conflicting requirements.

But what happens if, even with the best optimisation techniques, it is not possible to meet all these conflicting requirements? The extreme option is to set up an ECU with two completely different sets of parameters – one for normal driving and the other for when it is being tested for NOx emissions. By reading sensors such as the warning detector for the driver’s seat belt, movement of the steering wheel, the ABS sensors that compare the speeds of front and rear wheels and whether the collision avoidance system is isolated, it would not be difficult for the ECU to decide when the car is in the lab and when it’s on the road and to select the appropriate set of parameters. Many cars have a performance selector switch marked SPORT – NORMAL – ECONOMY; this would be adding a fourth position NOXTEST, selected automatically and secretly.

If selecting the least bad compromise between performance and emissions in a way that gives greater weighting to the latter is simply optimisation, the same cannot be said of switching to a completely different set of parameters when on test: this can only be described as fraudulent. However there is a spectrum of possible solutions that range between optimisation, through gaming the system to outright fraud. The greater the expectations and pressure placed on the design team, the greater will be the temptation to move along this spectrum.

We like to think that environment benefits, like reductions of CO₂ and NOx emissions, can be obtained by creative engineering without increasing the ownership cost of a product or reducing its performance in other respects. Up to a point this is true but there are always limits beyond which creativity cannot go. Because it is politically unpalatable to say that new motorways or HS2 will increase CO₂ emissions, low-carbon electricity generation will cost more than burning cheap coal or a third runway at Heathrow will increase NOx, there is a conspiracy of silence – we pretend that technology will provide a solution and we can have unfettered growth without the environmental downsides.

Perhaps we need to face reality. Much as politicians would like technology to decouple unconstrained economic growth from the drastic reductions in CO₂ and NOx we need for a sustainable and healthy life, this is not possible. Meeting the 2050 carbon emission targets is unlikely to be compatible with unregulated growth of air and road transport without fiddling the figures. Similarly stringent CO₂ limits and compliance with Euro 6 emissions standards may not be compatible with the sparkling performance and steadily falling motoring costs we have come to expect. If we are not prepared to accept compromises, we must expect more cases where reality fails to meet the theory.