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manolis
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Registered:
July 2003

Re: New stepless throttless valvetrain

Fri, 04 July 2003 07:32

mrshin wrote on Thu, 03 July 2003 20:16

Sure, I love it, in fact I want one - but isn't it rather similiar to what is already offered by BMW on their V12 engine?

There are many differences between BMWs valvetronic and the new system, even if both are continuously Variable Valve Actuation systems.

Valvetronic changes the lift of the valve together with the duration of the valve opening. For instance if you choose 1 mm valve lift, you get also 40 degrees valve duration. If you choose 8 mm valve lift, you get also 180 degrees valve opening duration. But if you like 1 mm valve lift with 180 degrees duration, it is impossible.
The valve opening also happens at about 100 degrees after TDC, so you absolutely need an improved additional continuous VVT of wide angle (they call it double vanos) in order to move the valve opening near to TDC at short valve lifts.
You need also additional strong restoring springs, one per valve.
You need also drive by wire and relevant electromotors, otherwise the engine cannot operate.
You need also a strong CPU and software to coordinate the subsystems involved.
And besides all these, valvetronic cannot control the exhaust valves, but only the intake ones.
Valvetronic includes a good number of parts (for instance it uses two reciprocating rollers per valve) and a good number of interfering joints conveying the motion from the cam lobe to the valve.

But if you try low revs with full load, the system is confused, as what is necessary is short valve lift (i.e. 1 mm) AND long duration (i.e. 180 degrees). Finally it necessarily works as a conventional valvetrain system (i.e. 6 mm valve lift with 150 degrees duration), losing the benefits it claims it offers: extreme turbulence and swirl and perfect homogeny due to the passing of the mixture through the narrow opening between valve and valve seat.
On the other hand, if you try to operate the engine above 6000 rpm, again there is problem, as the mechanism cannot operate safely there (heavy parts, high friction, wear concentration points etc).

The case becomes strange:
On one hand, a VVA system seems theoretically the perfect tool to make an engine operate efficiently in a wider range of revs, compared to conventional.
On the other hand the BMWs valvetronic VVA mechanism itself restricts the operational revs range, at low lifts due to inherent design characteristics, at high revs due to its strength, its internal friction, its inertia loads and its excessive wear.

So, even though BMWs valvetronic is an absolute commercial success offering better economy, lower pollution and improved drivability than conventional and two step VVA systems (like Hondas VTEC, Toyotas VVTLi, Porsches Variocam Plus etc), it does have a lot of weaknesses and limitations.

If a continuously VVA system could combine the power output of Hondas S2000 VTEC (120 PS per litter in normal production) with the economy, drivability, easy starting, low pollution etc of BMWs valvetronic (if possible without additional supporting systems like VVT, CPU, network, drive by wire, electromotors etc) then it would be a very good system indeed.

Thanks
Manolis Pattakos

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