Question for the engineers

[TwoSeven]

On an inline-4 bog standard 600. I know why the inlet closes at say 35deg ABDC - but why does it have to open at 15BTDC ?

edit: zorst closes 7deg atdc

if poss.. please show math.

[cowpoos]

it doesn't really....depends on the motor....whats it out of? that might help the techie's explain the state of tune...

[Motu]

Inertia,takes time to get things moving....and once moving to stop them again.

[idb]

What Motu said.
Fuel doesn't appear instantaneously in the cylinder when the valve is opened, it has inertia and needs to 'accelerate' from zero and move from the carburettor and into the cylinder.
So the valve has to be open long enough and soon enough to get the necessary charge of fuel into the cylinder before it is needed for combustion.

I'd be a shit teacher, but I know what I mean.

[Paul in NZ]

Dunno about the maths - no really I don't, but..

The inlet valve opens early because of something called 'overlap' which is defined as...

The amount of time, expressed in crankshaft degrees, that describes the window of time between the the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC.

Usually, this varies between zero degrees to as much as 70 to 90 degrees on some race motors. Most street engines will have 20 to 30 degrees of overlap and most performance cams will have 50 to 60 degrees of overlap. Increasing the degrees of overlap tends to move the powerband up the RPM band. Increasing the overlap can increase peak power, but ONLY if the exhaust system is properly designed to scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.

So Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event. The intake valve is opened during the exhaust event just before TDC, initiating the flow of the new charge into the combustion chamber.

As the exhaust gases are evacuated from the combustion chamber, a small but distinct low-pressure area is created on the surface of the piston head. By opening the intake valve earlier that TDC, the charge begins to fill this low-pressure area while exhaust gases exit. The low-pressure area on the head of the piston assists the fresh charge in filling the combustion chamber to its maximum capacity.

Valve overlap is designed into the engine and is most useful at higher speeds. At higher speeds, the extra amount of intake charge brought into the combustion chamber provides a substantial increase in available power. The amount of time that both valves are open in directly related to engine rpm. The higher the engine rpm, the shorter the amount of time that both valves are open. The angle of crankshaft rotation when both valves are open do so change, only the amount of time both valves are open varies. Thus at idle, the amount of time both valves are open is relatively long compared to that at top no-load speed.

[Motu]

The Goldstar cams I used in my B31 had 120 degrees of overlap - 65-85 - 80-55.So the inlet valve was closed with the piston almost halfway up the bore,the exhaust valve opening just past halfway on the power stroke...a fucking serious cam to run on the street.I couldn't run a ram tube as the fog it collected upset the mixture....the left leg of my jeans smelled of petrol.I used to run various exhaust systems and carbs,pistons,other cams.And still a first start kicker.

My XLV750 had 10 degrees overlap,valves opening and closing 5 deg before and after TDC,about the same as a Briggs & Stratton...how the hell did they get so much power out of it?

[Paul in NZ]

The Goldstar cams I used in my B31 had 120 degrees of overlap - 65-85 - 80-55.So the inlet valve was closed with the piston almost halfway up the bore,the exhaust valve opening just past halfway on the power stroke...a fucking serious cam to run on the street.I couldn't run a ram tube as the fog it collected upset the mixture....the left leg of my jeans smelled of petrol.I used to run various exhaust systems and carbs,pistons,other cams.And still a first start kicker.

My XLV750 had 10 degrees overlap,valves opening and closing 5 deg before and after TDC,about the same as a Briggs & Stratton...how the hell did they get so much power out of it?

I for one have never been able to get to grips with why some things work and others don't on certain engines. I know the B33 we 'tried' to improve with wild cams and a massive carb ran like shite... Later on we found the magneto was on it's last legs but as spotty youths, we assumed, spin the mag on the bench, spark appears - it's a good un.. complicated beasties magnetos - when they go wrong....

Had the same problem with the big concentric we tried, the engine never really shifted enough air through it to make it work properly, probably down to the wild cams and us not setting them up properly.

Eventually, I started listening to advice from trusted tuners rather than half reading Tuning for speed.. When my carbs on my hot atlas got so rooted it would not idle, I purchased smaller bodies (only new ones we could get) and a very kind man bored them out for me effectively creating smooth bore carbs which I didn't quite get then but they sure made a difference...

Funnily enough, I have resisted all the usual performance mods to my Guzzi and Triumph. Most of the tuners admitting these days that while you may gain a few bhp, there is a serious downside.... Funny old world..

[riffer]

So - if you were going to produce a GDI motorcycle engine, how would valve overlap work?

[Motu]

We are just pumping air then...but air still has mass,just don't have to factor in the fuel as well.Fuel injection has allowd intake runners to be tuned,this wasn't really possible with carbs,there was always a compromise.Check out a fuel injected car engine - nice intake manifolding the last 15 years.

[Paul in NZ]

We are just pumping air then...but air still has mass,just don't have to factor in the fuel as well.Fuel injection has allowd intake runners to be tuned,this wasn't really possible with carbs,there was always a compromise.Check out a fuel injected car engine - nice intake manifolding the last 15 years.

Yes, I'm certainly no expert on this but the idea of overlap is to get as much fresh charge into the chamber as possible. This is done by using the movement of the departing burnt charge pulling in the fresh charge etc etc.

Whether the charge is mixed in the inlet tract or the chamber is largely immaterial because it's the air that has the most mass to move. It should (I suspect) be more efficient because the air has no fuel vapour suspended in it but the challange would be to time the entry of the fuel so that it has enough time to mix with the air but not loose any into the exhaust ...

Then again, I know stuff all about it really... Ask me about flat screens, I know about them.... a little

[Ixion]

Port angles come largely into this too. The ideal is to have the incoming slug of mixture moving fast in a sort of spiral (if it moves too slow the fuel falls out of suspension), and shoot off down into the cylinder in a corkscrew direction that takes it away from the still open exhaust port.

And , in a perfect world, the exhaust port is being blocked by a pulse of gas pressure send back from the megaphone.

And the overlap is often partly determined by the inertia of the valve gear. Heavy valves, you just can't open them too quickly, the cam has to have a gentler ramp up (not to mention the dreaded silencing ramps!). Overhead cam engines tend to have less overlap because of this (cf Mr Motu's XRV750) . On an OHV pushrod engine, the first fraction of a second is taken up just flexing the pushrod, shoving the rocker around etc. Stiffer, less wobbly train on an OHC can react faster without getting all tangled up. Ditto for 4 valve heads. One overlooked advantage of these (overlooked cos motorcyclists don't care qany moe) is reduced fuel consumption - less valve overlap less fuel pumped straight out the zorst.

But, then emissions reduction stuffs all of this up.

'Twas always a black art. That's why I like two strokes. So nice and simple.

[Motu]

We've talked about this before,it's valve acceleration.The old Beeza had huge pushrods damn near a foot long,although they were alloy,and a couple of monster valves,and on the Goldy an 1 1/2in carb,that must of slowed gas speed down a lot...coming off cam was damn near stalling the airspeed in the port...and reversing it for sure...The cam was an unusual shape,a small base circle and long flanks with a flat tappet - it must of come up on the flank then levered the whole valve train up on the nose.I never valve bounced a B31 (well,I did with a broken valve spring) and I don't think the Goldy had problems,but 7,000 rpm must of been damn close to super nova stage.

But the XLV750 could make good power on lawnmower cam timing because of advances in valve weight,port shape,cam profile,and combustion chamber design.If we ever get to the point of having electronicaly controlled valves (I don't think it's possible with poppet valves) we will be able to have infinatly varable valve timing over the whole rev range....always ''on the cam''.

[riffer]

Wow. Great answers guys.

Could you control valves with electromagnetics? Or is it just not fast enough?

[idb]

Wow. Great answers guys.

Could you control valves with electromagnetics? Or is it just not fast enough?

I think I read somewhere that they are used in F1 engines.

[Motu]

No,they use pneumatic control to close the valves.An electronicly controled valve can't work very fast,they have been used on diesel engines,like about 5,000rpm max.They will have to use some other method of port control before they can be used in anything.....a 19,000rpm F1 engine? - no way in hell.