

Gill Sensors fuel flow sensor prototype fitted to an LMP2 chassis

Over the last few seasons of motor sport there has been a move towards the use of energy recovery systems with the view to making motor sport more environmentally friendly whilst aiding the development of greener technologies. Now in order to promote fuel efficiency the formula one 2014 rule changes will see fuel mass flow restricted to 100 kg/h above 10,500rpm and below a formula for the maximum fuel flow will be applied. In this databytes we consider how these restrictions will be enforced and look at the reason why the governing bodies are looking towards new technologies to police them.

Rather than using a mechanical restrictor, similar to the way that boost pressure is restricted in turbo charged engines the FIA have chosen to monitor flow rate through an onboard data logger and impose a penalty should a team exceed the permitted limits. This requires a reliable method of measuring the flow rate while not affecting the flow itself so an ultrasonic sensor capable of sampling fuel flow up to 4000 times a second has been selected. In this modern age where internal combustion engines are controlled using powerful electronics, capable of running engines at over 20,000rpm, you might ask why can’t the engine electronics themselves report the fuel flow, after all the cars run on an FIA specified ECU with FIA homologated software.

The control of an engine is complex but when you look at the fundamentals it’s possible to calculate mass flow rate considering injector opening times, fuel density, and the fuel pressure at which it is stored. In fact many ECU suppliers provide the ability to calibrate fuel based on fuel mass as well as injector opening time. The following figure shows the instantaneous fuel mass that is being calculated from the ECU fuel map at a given speed and engine load.

Varying fuel mass dependent on engine speed and load CLICK THUMBNAIL

It can be seen from the data (above) that the fuel mass being injected, injMassTotal, is also affected by other ECU strategies. For example when the engine speed drops fuel is cut as part of the over run fuel cut strategy. Later in the outing small spikes in injMassTotal can be seen and from further analysis of the data it can be determined that the spikes are due to the gear shift strategy which requests a down shift throttle blip to generate a torque reversal in the gearbox so that a new gear can be selected more easily.

For an engine to accurately deliver the required amount of fuel at each injection point there are a number of factors that need to be considered when calculating how long to turn the injector on for. Firstly the instantaneous fuel pressure needs to be closely controlled and monitored. In modern engines fuel can be injected directly into the cylinder head and to achieve this it’s necessary to pressurise the fuel to sufficiently to overcome the pressures seen in the cylinder head during the time fuel is being injected.

Also as its necessary to inject fuel at different points in the combustion cycle, to maximise engine performance, then its necessary to model the pressure differential that occurs across the injector tip and apply cylinder pressure injector flow compensation when calculating how long to open the injectors for. The accuracy of the model used will play a vital part in determining the accuracy of any fuel calculation performed by the ECU.

Additional compensation is also required for changes in fuel temperature and the effects of variation in battery voltage. Battery voltage compensation is relatively straightforward as injector manufactures will specify flow rates at different voltages albeit for direct injection applications this is not required as the injectors operate at voltages up to 200V to enable the precise control required by direct injection engines. In the case of fuel temperature compensation it is necessary to consider the heating effects that occur when the fuel reaches the injector tip. When the injector tip heats up, as the engine speed and load increase the fuel will expand. So at higher engine loads the injector needs to open for longer so that the correct fuel mass it injected into the cylinder.



Per Event and Cyclic Fuel Calculations performed by the ECU

The image above shows fuel calculations that are calculated as part of a standard direct injection application as this information is required to ensure the correct amount of fuel is sent to the fuel rail on a cyclic basis so that the fuel pressure is maintained at the required pressure. What should be noted is that the accuracy of these calculations is dependent on how well the compensation parameters are calibrated. So for example if an engine manufacture changed the position of the injectors then the calibration would need to change to account for different heating effects at the injector tip. This information would probably be determined by simulation and the accuracy of the compensation provided would be dependent on the resolution of the parameters used to calibrate it. Therefore all of these factors and dependency on multiple sensors mean that relying on fuel mass calculations to enforce a regulation would be fraught with problems. Sure if the teams can’t overcome these problems then their engines will be lacking in performance and reliability as being able to accurately determine how much fuel is being injected into the cylinders plays an important role in engine tuning. The problem here is effort required to manage this approach as the FIA would also need to check the engine calibration as well as the software used by each car. So it would appear that using an independent and homologated fuel flow sensor appears to be the only way that this regulation can be enforced so lets hope that the technologies being adopted are accurate and robust enough to stand up to the harsh environment of motor racing.

Written by Cosworth Electronics, first published in Racecar Engineering December 2013 edition.

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