Mazda CX-9 engineers ‘stalked’ SUV owners
BY BYRON MATHIOUDAKIS | 11th Jul 2016
With the aim of improving low-speed response as well as fuel economy, this involved engineers mimicking how drivers accelerated and braked while dropping off and picking up their children from school, before applying the data from their specially prepared prototype vehicle, to help tune the engine and transmission systems accordingly.
“We literally followed drivers around – with their permission of course – to see how they used their SUVs in their daily lives,” according to Mazda North America vehicle development engineer Dave Coleman, speaking to GoAuto at the launch of the second-generation CX-9 in Melbourne last week.
“A lot of focus went into achieving real-world fuel economy. If you focus on getting good (on paper economy) you often end up with a car that drives or delivers worse in the real world. So we focused on performance linearity and throttle response.” Mr Coleman said the results showed that sacrificing some torque at higher engine speeds in order to achieve stronger performance at lower engine speeds meant that drivers would not waste fuel by applying too much throttle pressure in the first place.
Mazda research showed that existing CX-9 owners were highly concerned about the fuel consumption from the Ford-supplied 204kW/367Nm 3.7-litre V6, making it one of the thirstiest petrol-powered seven-seater SUVs on the market. In contrast, the 170kW/420Nm 2.5-litre four-cylinder turbo is now rated the most economical in the US, partly as a result of the real-world tuning it has received.
In Australia, it delivers between 8.4 and 8.8 litres per 100km, compared to between 11.0 and 11.2L/100km for the outgoing model.
“So the trade-off with the 2.5-litre turbo is torque at higher-revs – from 4000rpm upwards – but that doesn’t matter because we found that, in the real world, all the action happened below 3000rpm.
“We found that out by taking a CX-5 diesel and loaded it up so it would weigh as much as a CX-9, and then followed school mums to soccer practice, to see how they drove… and then studied all the data.” While the new CX-9’s maximum power output jumps 16kW to 186kW using 98 RON premium unleaded compared to 91 RON petrol, it only comes in at over 4000rpm – an engine speed SUV owners rarely achieve, according to Mazda’s owner-usage research. Torque outputs remain unchanged.
Mr Coleman said that the turbo features a flow-control valve to cut lag and improve acceleration response from very low revs – restricting the size of the opening means the exhaust gases hit the turbine harder it is only beyond 1600rpm that the valve opens up fully for full exhaust flow to occur, providing more consistent and linear power delivery.
Other efficiency-enhancing measures include a substantially lower compression ratio compared to the normally aspirated SkyActiv engine (down from 13.0:1 to 10.5:1 though that is still high for a turbo), as well as the rerouting and re-burning of some exhaust gases into an air-to-water heat exchanger known as an EGR, to help keep combustion temperatures lower, which in turns helps it to burn fuel more economically.
“EGR doesn’t help in advertised (official combined average fuel consumption) specs but is very helpful in real-world economy and performance,” Mr Coleman explained.
