Engine mapping and blown diffuser explained

For the the British GP, Mercedes engined cars (McLaren, Mercedes GP, Force India) were able to use their fired-overrun. As this was pre-agreed with the FIA for reliability reasons. However Renault Sports request for their larger overrun throttle opening was requested after the event had started. Thus Chalrie Whiting decided that, as the technical regulations for the event need to be agreed before the event, Renaults request was inadmissible for this event. Thus they have to meet the original technical directive on throttle opening and not the 50% they had lobbied for. This leaves Renault having to run a mapping which is not optimal for reliability and Mercedes can run their mapping.
After much expectation on the effect of the 10% off-throttle limit, what transpired over the opening practice sessions brought more confusion than clarification. As practice got under way it transpires that the expected 10% limit was in fact not applied to all teams, nor was the dispensation to the different engine manufacturers communicated clearly to all the others. This brought much confusion to fans and media alike, as well as bringing a heated debate between Martin Whitmarsh and Christian Horner in the Friday press conference. Its been reported that Renault engines have been dispensation to run at up to 50% throttle when the driver is off the throttle pedal, and slightly less well reported that Mercedes engined teams are able to run a fired overrun.
The communication and political issues notwithstanding, the status is at least technically clear.

In order to prevent teams using off-throttle engine maps to continue to drive airflow over the diffuser for aerodynamic benefit, the FIA proposed a pair of changes to what’s allowed when the driver comes off the throttle pedal. Firstly the well known 10% limit on the throttle opening, but secondly a ban on injecting fuel into the engine when off the throttle. The intention of this pair of changes was to ban both hot and cold blown engine maps.
Of course this was the FIA proposal; the original date of the Spanish GP was delayed while the teams lobbied their cases to the FIA, giving their reasons why such changes were unworkable given the timescales and restrictions on development.

Now we need to understand what goes on within the engine when a driver lifts off the throttle and the subsequent effect that has on other aspects of the car. Unlike in road cars the driver in an F1 car doesn’t leisurely lift off the throttle and delay the braking phase. Instead the driver may be at near maximum revs, when he will simultaneously lift off the throttle pedal completely and hit the brake pedal hard for the initial downforce aided braking event. During the braking event the gears will be sequentially selected, further peaking revs as the car slows. This sudden closing of the throttles blocks off the inlet to the combustion chamber, but the cylinder will continue to pump up and down at a great rate. This creates huge stresses inside the combustion chamber and the vacuum created will suck air past the piston rings. This will rapidly slow the engine, creating too much engine braking effect, which in turns creates downstream stresses in the drive train and over-brakes the engine. The excessive engine braking effect will make the car nervous on throttle lift off, regardless of any subsequent aerodynamic effect.
So engine manufacturers find different solutions to ease the stresses and braking effect of the driver lifting off the throttle. In the seasons before EBDs there were several different strategies in place, the driver was able to alter overrun setting to tunes the cars handling, and driver switching between teams found the change in overrun settings needed some adjustment to both their driving style and sometimes with the engines settings. So overrun settings were already an issue before EBDs, and many strategies were already outside the intentions f the 10% rule.
Renault have been open and said their engine already runs open throttles on the overrun, this both eases the blow-by and stress issues, it also usefully cooled the exhaust valve, an alternative to using excess fuel to cool the back of the valve. This year the Renault sport are believed to be running as much as 90% open throttle on the overrun. This is what’s best known as cold-blown mapping. Earlier this season and through out free practice at Silverstone, the three Renault engined teams, had a distinctive loud overrun note, which continues briefly as the drivers picked up the throttle out of slow turns. As the throttles are open more than other teams, the induction noise is far greater.
Mercedes HPE, equally have their solution, this is the so called fired-overrun. When the driver lifts off, fuel continues to be injected into the engine and sparked within the combustion chamber. This offsets the engine braking effect created by the engine, giving a smoother transition from on throttle to the overrun when off it. As a result this means there is less engine braking effect. This gives Mercedes the freedom to define braking bias and KERS charging, without having to account for engine braking. Effectively decoupling the engine braking effect from the actual action of the braking system. As with Renault’s pre-EBD mapping Mercedes solution is analogous to the hot blowing mapping. At Silverstone the Mercedes engined teams had a particularly clean overrun sound. Where as Ferrari had far more cracks and pops as the engine slowed.
With both engine manufacturers having long established overrun strategies that have critical impacts on the basic engine design or the braking system, it will be hard to rapidly switch to a very strict overrun mapping as demanded by the 10% rule. Both manufacturers lobbied the FIA to be allowed to retain elements of these old overrun strategies, while still emasculating their current strategies. The FIA have been able to see the mappings used in 2009 through to the current day, as the code is held by the FIA since the advent of the single ECU (SECU). They’ve been able to see the engines have had these long established mappings, but also how they have become more aggressive since the EBD has been developed.
So the FIA relented and although we will commonly call this the 10% rule, the actual throttle will allowed up to 50% and some fuel can still be injected and burnt in the engine. This sounds like a climb down by the FIA and unfair to different engine manufacturers. But the unreported events at Silverstone this afternoon are fairer than the picture being painted by the teams and the media. Its true that Renault were given their greater throttle opening, but also Mercedes were given their fired-overrun, but these dispensations have been given to every engine manufacturer, so Ferrari could have more throttle opening or Cosworth could develop a fired overrun. As I understand you can one but not both of these options, so no 50%-open with a fired-overrun.

Although the communication and timing of these clarifications appears to be wanting, the final rules clarification meets the basic needs of individual engine suppliers, but still maintains parity between the four parties involved. There is no doubt this allows some secondary benefit of greater flow through the diffuser on the overrun, but this is still greatly reduced over what’s been raced already this year. So there will be reduced aero effect and no further arms race in developing these aggressive strategies. After the furore dies we have been left with w reasonable compromise on reducing engine effect on aerodynamics, before the fuller bans comes into effect with periscope exhausts next year.

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