The 2011 Formula One Guide - The Exhaust Blow Diffuser Technical Explanation

The objective of having air passing through the diffuser at the back, under side of a formula one car is to create more down force. This down force is created by the air under the car sucking the car down onto the surface of the circuit. It is actually a ground effect process. If the air is accelerated at the back, more air is sucked underneath the car causing a faster flowing, lower pressure, air stream creating more suction and therefore more down force. The diffuser is a series of expanding channels creating a venturi effect which accelerates the air through it.

With the banning of the double diffusers this year, Page 138 in The 2011 Formula One Guide, the constructors have been looking for ways to increase the effect of the single diffuser that they have at their disposal. One way of doing this has been to increase the airflow with the exhaust emissions. The problem with this is when the driver lifts his foot off of the throttle for a turn, the exhaust gasses are reduced just when he requires the down force most. A way to overcome this problem is to leave the throttle open, with or without burning fuel, so as to keep the air flowing through the engine and therefore the diffuser. The burning of fuel or not, while the throttle that the driver is not using is open, leads to two different approaches to the problem. If no fuel is being burnt then this is called cold blowing of the diffuser. If fuel is being burnt then it is hot blowing of the diffuser.

Obviously the hot blown diffuser is more efficient as the hot air is at an even lower density and that creates even lower pressure. What some teams were doing is to retard the ignition timing by around 35 degrees so that combustion was taking place while the exhaust valves of the engine were open creating a hotter airflow from the exhausts. This caused a higher than normal fuel consumption during a race. Lotus R are reported to have used around 10% more fuel per race than they did last year. The FIA were not happy with this extreme use of engines to aid aerodynamics so they issued a directive stating that the throttle opening would be no more than 10% when the driver was not using the throttle for acceleration. The implementation of the directive should have taken place at the Catalunya circuit in Spain. Teams argued reliability and expense to implement these changes and the directive was delayed to Silverstone. On Friday at Silverstone there was so much confusion as to what was to be implemented that nobody understood why the 10% had become 50% or that fired, fuelled, overrun was permitted.

So why all the confusion? Here we have to step back and take a look at what happens to a formula one car when a driver brakes. The car has three braking effects on it. One is the application of the brakes themselves, two is the KERS charging the batteries through the motor generator unit and three is the engine slowing down if it is not being given fuel and ignition. The engine still keeps pumping the pistons up and down even if the throttle is closed. If the throttle is closed no air is being drawn into the cylinder on the induction stroke and this causes massive stresses in the engine where air is sucked passed the piston from the sump. As the driver is also changing down gears the engine is doing maximum revolutions, called overrun, further exacerbating the stresses. To alleviate these stresses the engine manufacturers either leave the throttle open and/or fuel the engine creating combustion. Another consideration is that the car becomes very rear end unstable if the engine is fully braking on the overrun. As the drivers need cornering stability this is most undesirable. The open throttle and/or fuel burning leaves the car stable and the engine under very little stress. So long before the advent of exhaust blown diffusers the constructors were leaving open throttles and burning a modicum of fuel when the drivers were off the throttle. This practice was taking place in 2009 and the throttle open position was more than the FIA mandated 10% when the driver was not using the throttle. This the FIA could check with the mapping records they held as all the teams use a common ECU specified by the FIA.

So the 10% became 50% and fired overrun permitted depending on the constructors and engine manufacturers previous practice. This is the Friday night ruling by the FIA which may change during the race weekend.

This article should be read in conjunction with The 2011 Formula One Guide.

Norman Racer

I am a 65 year old male. An electrical engineer. My interest in Formula One started at a very early age and I have over 50 years of following the sport. I was lucky enough to be a race track marshal at Kayalami in the days it hosted F1. I am the author of The 2011 Formula One Guide