h a l f b a k e r yYeah, I wish it made more sense too.
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Recently I came to appreciate an LS1 V8 engine. The nicest
way to do this is to stand in the sun near someone who's
doing an awful lot of work on one, while carefully
depleting their beer supply. Every now and again you can
offer valuable advice such as "I'd have taped that hole up
BEFORE that
spring went in it..." Anyhow, I kept my mouth
shut about my confusion regarding the camshafts... until I
realized they were buried down inside... "it's OHV!" I
thought "like a massive lawnmower engine!" So instead of
the conventional arrangement in European and Japanese
4/6 cylinder cars of the valves being actuated by a cam up
in the head of the engine, the cam is down near the crank,
it moves pushrods which run up the cylinder wall and
rockers translate the movement down to the valves.
Essentially the cam is remote from the valves it actuates.
How remote could we make this? How about the dash?
Wouldn't it be nice to see your hot new cam in action?
Mechanical linkages between the cam and the valves might
get a touch... complex if the cam were that far away, but
we don't need those. So, we turn the cam, lets go with a
very carefully controlled brush-less electric motor. The
cam acts on followers which move hydraulic pistons. The
pistons move hydraulic fluid through nice flexible lines to
the complementary pistons which act on the valves. By
messing around with the piston sizes you can do exactly
what the rockers do, in terms of the ratio of valve lift to
cam profile. Now you have a cam spinning in a nice clear
case on your dashboard.
A brief read around the fairly involved subject of hydraulics
suggest they have no problem with the pressures, the
absolute flow rates and so on. The speed of acceleration
might be an issue. You could make the whole system one-
way flow if you needed, with oil exiting at one end and
refilling at the other. You could also control the valve lift
by changing the volume of the hydraulic push rod. If you
put the cam back in its original location you can use the
hydraulics to change lift, and with some clever valving,
duration. If you do all that and put the cam on top of the
engine, you've got Fiat's multi air system.
Visible V8
https://www.google....FIKHVawBj8Q_AUICCgD
Moves like a real engine, but doesn't burn fuel. [Vernon, Jun 13 2016]
[link]
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Lots of WW1 aero engines had exposed rockers and valve springs, with pushrod actuation. Building an engine with an OHC engine like that, incorporating hydraulic tappets, would probably be easy enough ... but roller bearings would probably be better than journals, to avoid oil being flung everywhere. |
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When I first heard about hydraulic tappets/HLA's
whatever, I thought they essentially were hydraulic
pushrods. Instead they're just there to avoid all that
messing about with shims, and, I suspect the collapsed
tappets provide easier cranking and the attenuated
lift/duration makes for better warm up characteristics. |
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The Multiair system essentially is a collapsible hydraulic
push rod. But they fitted it at the top. The redline is
suspiciously low for a DOHC engine. in the mid 6k range,
which is well within pushrod V8 territory. I think they
could safely ditch the OHC. Since all the timing and lift
variation can be controlled by hydraulic valving, you
could just go with a gear driven cam down near the
crank, or hell, just put lobes on the crank and only use
every other rotation. |
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