I was reading an article on Austin Healeys about how AH considered using a Rolls Royce engine to continue the production of the big Healeys.
In the article they refer to the engine as being an inlet over-exhaust 4-1litre Rolls-Royce engine.
While I am sure the 4-1 is 4.1 liters I have no clue what an inlet over exhaust engine is.
Furthermore the article goes on to say But the breathing on an inlet-over-exhaust engine is limited by the compression ratio- you can’t go very high. And although with the ‘F’ head you can get a lot through the inlet valve, you can’t get it out, as the exhaust gases have so far to travel from the combustion chamber to the exhaust valve.
Can anybody explain what an inlet over exhaust engine is and why the exhaust gas has so far to travel?
“F” discribes the shape of the combustion chamber. The verticle line is the cylinder bore that the piston operates in, the top line is the intake port, and the bottom line is the exhaust port. The valving is at or toward the right hand ends of the horizontal lines. The horizontal lines create volume that the piston can’t sweep, which is what limits the compression ratio that can be attained. Since the piston can’t reduce the combustion chamber volume to a very low value on the exhaust stroke, a lot of exhaust is left in the cylinder. While this does reduce NOx emissions, it also limits power.
In a “normal” combustion chamber, the valves must share the single “roof” of the cylinder head, which limits them to half the diameter of the cylinder, at max. The F combustion chamber eliminates this limitation.
Hit submit too quick… goofy looking critter! So what advantages does it have to account for the complexity of apparently needing two valve trains due to having intake valves in the head and exhaust valves in the block? (Not to mention that compression issue.)
Thanks to all, this was kind of what I had envisioned in my mind they were describing, but I couldn’t work out the kinks.
Beats the shit out me. I guess it is an improvement over a straight flat head design, but a hell of a lot more complex. Like a buddy of mine says about British Cars. More parts doing less.
Lucas, inventor of the self dimming headlight.
The three positions on a Lucas headlight switch? Dim, flicker and off.
What is the motto of Joseph Lucas Ltd.? Home before dark.
In Greek mythology Lucas was the prince of Darkness.
If Lucas built torpedoes wars wouldn’t start either.
In 1975 Lucas decided to diversify. They came out with a vacuum cleaner. It was the only product they ever built that didn’t suck.
Dad was the Canadian Service for Lucas in the 60’s.He always complained that the attitude of head office in U.K.was if it’s alright in U.K. it should be O.K. anywhere. I remember when Lucas had a windshield washer that the pump melted in the Glass! Bottle when windshield washer was added.or if you didn’t use antifreeze the bottle cracked. The U.K. attitude was “Why would anyone use a windshield washer in freezing temperatures!!!”They finally changed the plastic in the pumps and changed to plastic bottles about 4 years later.
Mainly the problem is that because the valve is in the block, the piston can’t go all the way up the block, as it can’t go up past the valvle, and so the compression ratio (max cylinder volume / minimum cylinder volume ) is low.
Not sure that there “distance from valve to combustion chamber” thing is correct, its all combustion chamber near the values. Maybe its really the side exit, the gases won’t turn the corner, they are going up and down with the piston… gases have momentum, they don’t turn just because a valve is open.
The extra surface area imposes two other disadvantages (relative to a modern automotive engine with all valves in the head directly above the piston):
more heat rejection to the surfaces of the combustion chamber during the power stroke, reducing efficiency; and
greater emissions of raw hydrocarbons. Due to heat transfer, the flame front is extinguished a fraction of an inch from all of the combustion chamber surfaces, leaving a thin layer of unburned fuel-air mixture which then gets swept out during the exhaust stroke. More surface area means more of this unburned mix left behind after the power stroke.
