Potentially Stupid Question - Overhead Cam Engines
Discussion
Another reason is OHC makes it easier to get power from smaller capacity engines which require multi valve heads. On a 4" bore of the LSx engine you can get away with larger 2 valve heads and as stated earlier, this is a simpler, more compact design.
My 6.2 V8 revs to 7000rpm so not a lazy engine at all.
My 6.2 V8 revs to 7000rpm so not a lazy engine at all.
Super Slo Mo said:
This is the basis of the Fiat Twin Air engine, albeit it's all done in the cylinder head. No doubt it could be achieved by a cam in block approach instead, and the technology allows for a huge amount of variables with regards to cam timing, variable lift etc.
It would be a complete re-engineering of the engine concept though, whereas the Twin Air approach uses a conventional block and just bolts the new head onto it.
Yeah I was aware it had been done (by various people I believe?) with OHC engines but with the desire to reduce engine height in order to keep bonnet lines down whilst maintaining the mandated hard-point gap, I'd have thought an OHV engine with really good valve control would be quite desirable. It would be a complete re-engineering of the engine concept though, whereas the Twin Air approach uses a conventional block and just bolts the new head onto it.
kambites said:
Super Slo Mo said:
This is the basis of the Fiat Twin Air engine, albeit it's all done in the cylinder head. No doubt it could be achieved by a cam in block approach instead, and the technology allows for a huge amount of variables with regards to cam timing, variable lift etc.
It would be a complete re-engineering of the engine concept though, whereas the Twin Air approach uses a conventional block and just bolts the new head onto it.
Yeah I was aware it had been done (by various people I believe?) with OHC engines but with the desire to reduce engine height in order to keep bonnet lines down whilst maintaining the mandated hard-point gap, I'd have thought an OHV engine with really good valve control would be quite desirable. It would be a complete re-engineering of the engine concept though, whereas the Twin Air approach uses a conventional block and just bolts the new head onto it.
R8VXF said:
The new LT series of engine have VVT and also cylinder deactivation to help with efficiency, coupled with dual injection (both direct and port). With the VVT you still have the fixed Lobe Separation angle though.
Yeah, VVT in OHV engines is nothing new. As I understand it the problem with high revving OHV is the extra mass of the components the cam shaft is moving requires stronger valve springs to avoid excessive bounce leading to increased parasitic losses and wear; and any flex in the valve train components leads to poorer control reducing the potential volumetric efficiency. Obviously this can be aided by using lighter, stiffer materials for the push-rods (is carbon fibre viable?).
I think hydraulics would be significant better though, if it was commercially viable. In fact I'd go so far as to say that hydraulic push-rods could potentially completely negate the advantages of OHC engines, leaving OHV as a clearly better solution? I suppose high-end engines may ultimately end up going to full servo control of valves anyway, negating the need for a cam-shaft completely. Or everything will just switch to electric motors which don't have the problem.
Edited by kambites on Monday 27th July 09:39
Or just get rid of the camshaft completely like Konigsegg are trying to do: http://jalopnik.com/what-its-like-to-ride-in-a-car...
TheAllSeeingPie said:
Or just get rid of the camshaft completely like Konigsegg are trying to do: http://jalopnik.com/what-its-like-to-ride-in-a-car...
Camless engines have existed in marine and industrial applications for years, but it's only now that the tolerances and control can be made small enough and precise enough to consider them in such a compact unit as found in a carTie that in with laser ignition (replacing spark plugs) and it'll be very interesting to see what additional lease of life the internal combustion engine will get in terms of power, efficiency and cleanliness
HustleRussell said:
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
Err, yes. Alfa Romeo 1954~1994; twin cam, hemi combustion chambers, cross flow head, forged crank etc. ended up being the first production car engine to use VVT. Alfa had lots of firsts, as does FIAT now (common rail derv, twin air head, etc). They mucked about with cylinder deactivation on the Alfa33 (never went to production)Lost there way a bit in recent decades, hopefully about to make a nice comeback.
HustleRussell said:
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
The Buick 215 / Rover V8.Can I have a go .. an OHC engine has less reciprocating mass (just the cam goes up-down) but is potentially more complex - the pushrod mechanism has to be replaced by some other method of translating the crankshaft motion into an up-down motion. They are essential for F1 engines (20k+ RPM) which have pneumatically-operated valves.
Is there any reason not to use linear actuators? http://www.techdrives.co.uk/linear-actuators.html
R8VXF said:
Is there any reason not to use linear actuators? http://www.techdrives.co.uk/linear-actuators.html
Not fast enough I would have thought. You need to be able to move a short precise distance in milliseconds to open and close a valve.budfox said:
This could be an embarrassment but I'm going to ask anyway.
Why don't all engines use overhead camshafts? What is gained by the complexity of a camshaft/pushrod/rocker system? Surely OHC is simpler, cheaper, more efficient, more reliable etc...
Might have been asked a thousand times before, but there you go.
You have it the wrong way round. OHC makes engines taller, longer and raises their centre of gravity. And adds a more complex pulley arrangement.Why don't all engines use overhead camshafts? What is gained by the complexity of a camshaft/pushrod/rocker system? Surely OHC is simpler, cheaper, more efficient, more reliable etc...
Might have been asked a thousand times before, but there you go.
This is especially noticeable in Vee engines. A current Chevy Ls V8, such as in the new Corvette, makes 430bhp, 450lb ft (or something like that), can do almost 30mpg. But is physically smaller and lighter than most Jap/Euro DOHC V6 engines that make less 200bhp.
You also have only 1 cam vs 4. So far less complex and a lot cheaper to upgrade.
In terms of OHC. The real advantage comes from multi-valve engines. Driving more than 2 valves per cylinder, while possible with an OHV setup, is quite complex. DOHC solves this and allows easy use of 4 or even 5 valves per cylinder.
The advantage here is curtain area. As you rev an engine higher, it'll need more air going in and out. You can do this with a 2v head, but it generally involves large valves, which then need heavy springs. This will get it breathing well at high rpm, but can make it lumpy and less tractable down low (think how lumpy tuned classic American V8's sound). It also isn't good for mpg or emissions.
Multi-valve allows precise control to open 4 valves a small bit, rather than 2 valves a lot to get a similar cfm result. This means better mpg/emissions, more tractable down low and a broader powerband. A nice example is Honda's K20 VTEC, than can even adjust the cam timing. It has a smooth powerband from ticker to 9000rpm.
An OHV engine, while it could physically run at these rpm's, is limited to more like 6000-6500rpm for practical road purposes, due the above points mentioned.
Inline 4 and 6 and even 3 cylinder (more common today) all benefit from good DOHC setups. But in Vee terms, the Yank LS from GM and the Hemi from Chrysler are smaller, lighter and more compact than any Euro/Jap DOHC Vee engine and generally out perform them all too.
Mr2Mike said:
AKAIK F1 engines have pneumatic valve springs, but the valves are still actuated by camshafts.
I think it's all about getting the valves quickly and firmly closed without the "bounce" or "float" that can tend to occur due to a metal spring.Ducati's desmodromic valve system similarly slams the valves hard shut. One cam lobe/rocker to open the valve and another lobe/rocker to shut the valve.
300bhp/ton said:
You have it the wrong way round. OHC makes engines taller, longer and raises their centre of gravity. And adds a more complex pulley arrangement.
This is especially noticeable in Vee engines. A current Chevy Ls V8, such as in the new Corvette, makes 430bhp, 450lb ft (or something like that), can do almost 30mpg. But is physically smaller and lighter than most Jap/Euro DOHC V6 engines that make less 200bhp.
450bhp (460 if you get the Z51 pack). 29 mpg is possible thanks to direct injection and cylinder deactivation which shuts down fuel to cylinders 1, 4, 6, and 7. And it also has variable valve timing too.This is especially noticeable in Vee engines. A current Chevy Ls V8, such as in the new Corvette, makes 430bhp, 450lb ft (or something like that), can do almost 30mpg. But is physically smaller and lighter than most Jap/Euro DOHC V6 engines that make less 200bhp.
amusingduck said:
Does anyone know whether they made an LS engine with more than 2 valves per cylinder?
If not, why not?
I think there was briefly a DOHC 32 valve LS V8. As far as I know there's never been more than 2 valves per cylinder on the push-rod engines, I suppose it's just too hard to make paths for 32 pushrods through the block and head; I don't know whether you could actuate 2 valves from one pushrod somehow. If not, why not?
300bhp/ton said:
are smaller, lighter and more compact than any Euro/Jap DOHC Vee engine and generally out perform them all too.
Sorry, but B*ll*cks!The 455bhp/620Nm LS V8 is 6.2l, that's a reasonable 100Nm/l (ok, but nothing special these days) and a lame 73bhp/l (awful)
Also, whilst they are compact for a 6.2l V8, they are NOT smaller/lighter than a small capacity euro V6 engine
However, they are a very cheap, robust way to 455bhp, but come with massive economy penalties which simply are not viable in the European market (hence no European OEM uses one)
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