Engine bore and stroke

[aero expert 807]

if an engine had a 98mm bore [maximum size alowed] what would the stroke be? I have looked everywhere and can't find any info.
Also, what be the most effiencient combination to achieve 2.4 litres?
I think higher bore, lower stroke would be better since the engine revs so high
Thanks for your time

[Lycoming]

its a fairly simple calculation. displacement (2.4 L) is the swept volume.

d/2 = r, so the area of a circle as a function of diameter is: A = pi * (d^2/4)

swept volume for cylinders then is A * stroke, so we get:

2400 = 8 * stroke * pi * (9.8^2/4)

solve for stroke and you get 39.8 mm

Yes, larger bore is better for high rev operation, which is typical for an F1 engine. one reason for this is it results in lower piston speed.

[matt21]

An additional bonus wehen you have a short stroke is, that you can lower the crankshaft due to shorter crank pins.

[coaster]

Not clear on that algebra lycoming, but basically it is the formula for a cylinder, radius squared multiplied by 3.14 (pi), put that in the memory (M+), divide the displacement by the number of cylinders then multiply by 1000. Divide that by the memory (RM) and wallah!!!! =D>

[olefud]

Another advantage of a larger bore is the accommodation of larger valves with more favorable positioning. This may be offset somewhat in non-boosted engines by the need for a rather small TDC volume for a reasonable compression ratio.

[Ted68]

Another formula

(((Bore squared).7854)Stroke)#Cyl=Displacement

[spacer]

Be carefull with the "the bigger the bore the better" line of thought, there are limits to the advantages of a wider bore / shorter stroke.

Current F1 bore/stroke are probably dictated by engine speed (the high reciprocating forces and need for ultra wide valves to allow some useable volumetric efficiency being the problems).
The large bore / short stroke has disadvantages in terms of mixture burn; the wider the bore the more crankshaft duration one needs to burn the mixture, with fuel burning speeds as a given for a certain set of engine operation parameters, this forces more ignition advance and thus more pumping losses and stress put on piston/rod.
Another big disadvantage is the actual space that's available inside the combustion chamber - piston to valve clearance being important. This both limits the cam lift and geometric compression ratio. I'm quite positive current F1 engines run at CR's much lower than the engineers would ideally like to with the current fuel specs.

[Tommy Cookers]

Be carefull with the "the bigger the bore the better" line of thought, there are limits to the advantages of a wider bore / shorter stroke.

Current F1 bore/stroke are probably dictated by engine speed (the high reciprocating forces and need for ultra wide valves to allow some useable volumetric efficiency being the problems).
The large bore / short stroke has disadvantages in terms of mixture burn; the wider the bore the more crankshaft duration one needs to burn the mixture, with fuel burning speeds as a given for a certain set of engine operation parameters, this forces more ignition advance and thus more pumping losses and stress put on piston/rod.
Another big disadvantage is the actual space that's available inside the combustion chamber - piston to valve clearance being important. This both limits the cam lift and geometric compression ratio. I'm quite positive current F1 engines run at CR's much lower than the engineers would ideally like to with the current fuel specs.

Let's agree that current B:S ratio around 2.5 is interesting and unprecedented (and I do agree with some of the above)

surely VE is unprecedentedly high around 120%
larger B:S ratios automatically tend to ample valve area
(with B:S rise valve area rises faster than demand on valving via rpm rise)

we have had special fast combustion fuel for 20 years in F1 (ign advance is not abnormal)
if combustion speed was a problem we would have twin spark plugs
so pumping is not abnormal (the stress described is in principle helpful)

CR is nominally around 14

current fuel spec (right now) has no max octane limit (interestingly)
high octane may not be needed anyway
(if spark-initiated combustion outstrips the (delay) time for detonation to start, there is no detonation problem, 1960s Honda GP winning motorcycles needed only 65 octane around peak power rpm for this reason (granted only 44mm bore))
(diesel has a delay time, so does petrol)

sometimes I can't believe how well (brilliantly) F1 engines do their special job !

[alelanza]

if combustion speed was a problem we would have twin spark plugs
so pumping is not abnormal (the stress described is in principle helpful)

Not possible:

5.8 Electrical systems:
5.8.1 Ignition is only permitted by means of a single ignition coil and single spark plug per cylinder. The use of plasma, laser or other high frequency ignition techniques is forbidden.

http://www.formula1.com/inside_f1/rules ... 4/fia.html

[strad]

You want the rod length to stroke ratio to work out to where the crank pin is at 90° at ignition, so all the power stroke is used to push the crank down..if you have it past 90° part of the power is lost pushing past BDC,,and by definition is trying to push it up..if you fire too far before 90° part of the power is lost pushing the pin sideways instead of down. It is part of what makes a small block Chevy work so well to hot rod.
Somewhere close to 1.5-1 is best.

[aero expert 807]

Thanks for the responses everyone!
The tricky thing is to find the ideal B:S ratio to produce enough power,reliability,and efficiency

[strad]

I'm tellin ya...Bore Stroke ratio is not as important ans rod length to stroke ratio.
I'm not a math wiz but I think you can achieve that 90° position of the crank pin thru other than 1.5/1.
You need the crank pin horizontal for the reasons mentioned before.

[aero expert 807]

I'm tellin ya...Bore Stroke ratio is not as important ans rod length to stroke ratio.
I'm not a math wiz but I think you can achieve that 90° position of the crank pin thru other than 1.5/1.
You need the crank pin horizontal for the reasons mentioned before

I understand what you are saying I was just speculating abouut it from B:S ratio point of view only.

[Tommy Cookers]

the short rod means that the piston spends (ralatively) more time at TDC, then accelerates away faster

ideally combustion should all happen close to TDC (ie all the heat added to fully compressed air), then expansion
the combustion speed is roughly the same whether the engine is doing 1500 rpm or 15000 rpm

so high rpm engines always have short rods, basically so that the piston waits long enough for the combustion
(despite short rods giving higher accelerations, ie higher stress and ring difficulties)
slow engines don't need short rods, and don't have them because their greater angularity gives more side thrust/frction
(except when short rods result from stroking-out an existing engine for more capacity)

F1 B:S ratio causes high heat loss to coolant, so this ratio would be very bad at partial powers ie in a road car
2014 F1 mandates a much lower ratio for (fuel) efficiency, rather than the present max power with unlimited fuel

[strad]

the short rod means that the piston spends (ralatively) more time at TDC, then accelerates away faster

ideally combustion should all happen close to TDC (ie all the heat added to fully compressed air), then expansion
the combustion speed is roughly the same whether the engine is doing 1500 rpm or 15000 rpm

so high rpm engines always have short rods, basically so that the piston waits long enough for the combustion
(despite short rods giving higher accelerations, ie higher stress and ring difficulties)
slow engines don't need short rods, and don't have them because their greater angularity gives more side thrust/frction
(except when short rods result from stroking-out an existing engine for more capacity)

F1 B:S ratio causes high heat loss to coolant, so this ratio would be very bad at partial powers ie in a road car
2014 F1 mandates a much lower ratio for (fuel) efficiency, rather than the present max power with unlimited fuel

All true, also the stroker motor has more of an arm (crank throw),to sling around which limits how fast you can spin it.