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MultiAir does MultiJet for petrol March 8, 2009

Posted by Richard Aucock in What I learned today.
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Diesel has devoured the bulk of car maker development budgets in recent years.

It’s been a quick and dirty way for them to reduce parc CO2 emissions. Petrol’s been left lagging. Fiat’s helping it catch up, and giving us a new acronym in the process. MultiJet for diesel, meet MultiAir for petrol.

As the company that invented common rail injection, now de rigueur for diesel, MultiAir is thus maybe quite significant (not least because it’s not as dirty as diesel). But what on earth is it?

A way to make petrol engines 25 per cent more fuel efficient, that’s what. God knows, they need it. Geneva was the first signs that development budgets are switching to petrol. How they’ve some catching up to do. The weediest 1.2 Grande Punto can’t even average 48mpg. The zappy 90bhp 1.3 turbodiesel? Nearly 63mpg. Plus 20g/km less CO2. That’s a big difference (circa 25 per cent in fact), even if the problem is that you do pay for it.

Now then. How it works. Fiat told me that if you want to enhance diesel performance and emissions, you need to control how much fuel you inject into the cylinders. It’s down to how accurately you can do this, too.

For petrol, though, the trick is to play with not the fuel, but the air being injected.

Normal engines have a ‘dumb’ intake valve. This can only open or close. How much air goes into the cylinders depends on the throttle valve, further up the air supply chain. This is (says Fiat) wasteful. What you should be doing is controlling the intake values themselves, electronically. At source, rather than further up the chain, so to speak. How to do it cheaply, though? That’s what’s been keeping car companies busy, apparently, since the 1980s.

Fiat’s solved this. MultiAir is easy, cheap, variable valve actuation, giving full independent control over what the intake valve does. Hurrah. Diesel eco without the diesel cost, plus cheaper fuel to boot. This is big stuff. But this realisation didn’t come before I’d interpreted a tech-heavy press release…

Token technical image that next to nobody will understand, not least me

Eventually, I found out MultiAir uses a piston connected to the intake valve. It’s moved by a cam, but the clever part comes because it’s connected via a hydraulic chamber. A solenoid valve controls this. This can have two states – open or closed. Now, then:

• Solenoid closed? Oil behaves a like a solid body. Intake valves do what the mechanical cam says.

• Solenoid open? Bingo: intake valves decoupled from intake camshaft! They close instead under valve spring action. (This is why Fiat also fitted a hydraulic ‘brake’, for soft and controlled valve closing…)

So, what tricks does it offer? Well, the solenoid is always closed for maximum power. But for low-rev torque, independent operation comes in. It opens near the end of the cam profile, meaning the values close early – trapping as much air in the cylinders as possible.

However, for part load, it opens much earlier, which does all sorts of clever things to airflow. This boosting torque. Or, it can be opened later, boosting ‘higher-in-cylinder’ turbulence. These two modes, called ‘MultiLift’, can be deployed in the same stroke, which is the really, really clever part. And which is why it’s taken so long for the ECU engineers to map…

It’s not just for petrol, either. Potentially, it reduces diesel NOx emissions by 60 per cent, and taking 40 per cent of unburned hydrocarbons out of cold start emissions. Indeed, Fiat says that this is just the start. MultiAir could even see petrol and diesel engines unified, rather like Mercedes and VW are proposing with DiesOtto.

The first MultiAir will be a 1.4 Alfa Romeo later this year. Fiat will also fit it to its new two cylinder engine, coming to the 500 in 2010.

No need to hedge bets on the fuel of the near-future, then. Seems it’ll be a bit of both…