Ferrari Mailing List: Re: Elementary engine question -- NFC

[Doug and Terri Anderson (dnt]

>>Britt notes “. . . .[modern cars] have variable valve assist of some sort and quite a few of them will open the valves to remove the compression and allow the engine to freewheel quite a bit further all in the name of fuel savings.

 

“Variable valve assist” generally moves cam shaft timing geometry and the valves are opened mechanically (unless the engine is an F1 motor).  The variation is changed by either oil pressure via solenoids or electronically (tho I don’t know of one that is now 100% electronically controlled – unless you recall the time bomb 1981 Cadillac Fleetwood V8’s 8-6-4).

 

In most new engines, hanging the valves open will cause a bangety bang sound as the piston collides with the valves.  Most new engine designs are an “interference” engine, have higher compression ratios, and therefore a very tight squish area chasing a closing valve.  Should a valve hang open as in a 308’s F106 motor due to a smog pump seizure stopping the aft exhaust cam – bangety bang and the hills are alive with the sound of money.

 

Since 1996, newer OBDII engines, to meet CAFÉ standards, may shut off fuel to one or all cylinders.  Most notably this happens when the engine detects an ignition miss – that program measures the angular velocity between individual cylinder firings and if it picks up an undue lag it packages that info enough times within a set of parameters & will shut off the fuel to that cylinder.  Then the PCM program commands the malfunction indicator lamp (MIL) or in old terms – the Check Engine Light (CEL) to blink.  A blinking MIL is bad news indicating an ignition miss and your owners manual states to STOP now.

 

But the above goes past our original question – does the engine shut off all fuel to the cylinders when coasting.  Dunno.  I do know that newer cars in the last few years offer or simply stick you with an “Engine shut down at red light” optional program.  These are designed to save gas but what about stop and go traffic?  What about battery life?  What about starter life?  And how is drivability affected?  I drove the new Porsche Panamera with the option set ON and found it not seamless as sales brochures, the company, and sales staff bleat.  The option was quickly shut off.  Ahhh – back to normal.

 

I rest my case.

 

DOUG