Engine bore and stroke

[marcush.]

as far as i understand bore diameter is fixed in the current rules -so there is no leeway for optimising stroke.

The current bore diameter was a function of the revs that were rocketing before the revlimit of todays was set...so potentially the bore diameter todays is less than optimum for the revs we see.

A big bore diameter has one real issue - flametravel is a fixed speed event roughly ..so having a big chamber diameter is not what you want as you need time to burn the fuel...

[Tommy Cookers]

...... surely bore is arguably more than the optimum for the current rpm ?
(the engines were designed/developed for more rpm, the present reduced rpm is for long life)
(present rpm could tolerate a longer stroke and slightly smaller bore if life was reduced to the traditional level)
(this should have a little more friction and less heat loss, but less valve area)

combustion speed must be ok now as it was ok a few years ago at 2000 more rpm ?

F1 has always tried to avoid fuel limiting for efficiency, until now (2014) !

[olefud]

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.

I’m having trouble following this. Could you elaborate a bit, i.e. is “ignition” firing the plug or complete flame propagation? It would seem that full fuel charge combustion is desirable as early as possible (sans detonation) at power TDC. The work and related power is a function of the pressure on the piston times distance traveled –so it would seem that losing a bit to scuff is better than no work at all during that portion of the power stroke (sort of like halitosis relative to no breath at all). Also it would seem that any respectable cam would open the exhaust prior to power BDC for blow down to avoid working against residual combustion pressure on the exhaust stroke.
Switching gears, it’s true that a large bore to stroke results in a thin, wide combustion chamber at TDC. However, if a problem, the rate of flame propagation can be enhanced by providing squish areas which, in effect, add convection flame propagation to the normal flame propagation rate.

[aero expert 807]

as far as i understand bore diameter is fixed in the current rules -so there is no leeway for optimising stroke.

The current bore diameter was a function of the revs that were rocketing before the revlimit of todays was set...so potentially the bore diameter todays is less than optimum for the revs we see.

A big bore diameter has one real issue - flametravel is a fixed speed event roughly ..so having a big chamber diameter is not what you want as you need time to burn the fuel...

From fia regulations:
ARTICLE 5: ENGINES AND KINETIC ENERGY RECOVERY SYSTEMS
5.3 Engine dimensions:
5.4.1 Cylinder bore diameter may not exceed 98mm
this implies that cylinder bore is flexible

[strad]

I’m having trouble following this. Could you elaborate a bit, i.e. is “ignition” firing the plug or complete flame propagation?

You of course require ignition lead (advance).
If you were to wait until, then it would be too late huh.
Ya just don't want to waste energy pushing on the pin too much before 90° or after BDC.
Which begs the question...How much lead do they run at darn near 20,000 rpm?

[marcush.]

oh now i remember ,it´s not the nore ,but the bore spacing that is fixed -so the outer dimensions of the engine are pretty much fixed by this.

[Tommy Cookers]

From fia regulations:
ARTICLE 5: ENGINES AND KINETIC ENERGY RECOVERY SYSTEMS
5.3 Engine dimensions:
5.4.1 Cylinder bore diameter may not exceed 98mm
this implies that cylinder bore is flexible

good point !

I guess that they are all the same bore and stroke now (we shouldn't have to guess) ?
Renault were a smaller bore surely ? how/when in thie 'frozen engine' era did they make the change ? on durability grounds ?
minimum bore spacing rules were presumably written around these engines (as were the rpm rules ?)
if they still use the smaller bore they have done well on durability, their stresses would be higher
heat loss to coolant would be lower, but frictional losses a bit higher, and valve area less

nearly 30 years ago Honda had an even higher B:S ratio in their NR500 GP motorcycle
this had wildly non-round 'cylinders' with 8 valves etc (to subvert 4 cylinder limit), and was not great

current B:S results from (unprecedented) decades of engine rule stability after the elimination of turbos
totally logical, but extreme B:S would not work (with single ign) without high combustion speed fuel (last 20 years)
presumably this factor prevents even higher B:S, and normal fuel would not support current B:S

the current F1 engines are brilliant and logical (enabled by their remarkable gearboxes, sorry autogyro!)
their valve sizing breathes so well that they really use their revs ie the gearboxes enable this 'half a powerband' setup
(historically engines eg V12 did not breathe and/or combust well enough to use well their higher rpm potential)

[Tommy Cookers]

You of course require ignition lead (advance).

Which begs the question...How much lead do they run at darn near 20,000 rpm?

great question !!

(FWIW my guess is nothing abnormal, big advance is only needed when combustion is inconsistent ?)
(we shouldn't have to guess though)

EDIT this means to the author of this post 'what is the ignition timing in degrees ahead of TDC ?'

eg the Cosworth SCA used up to 72deg 'a sure sign of inconsistent combustion' (it won thousands of races)
..... this I call abnormal

[marcush.]

flame travel is more or less a constant but as your rpm rises your time to ignite and completely burn will get shorter and shorter so it seems quite obvious one would need to advance timing with rising rpm to get the pressure peak back to reasonable crank angles.

[strad]

flame travel is more or less a constant but as your rpm rises your time to ignite and completely burn will get shorter and shorter so it seems quite obvious one would need to advance timing with rising rpm to get the pressure peak back to reasonable crank angles.

Yeah..That's what I meant...A small block running on gas that turns 10,000 rpm in a dragster needs about 45°.

[alelanza]

as far as i understand bore diameter is fixed in the current rules -so there is no leeway for optimising stroke.

The current bore diameter was a function of the revs that were rocketing before the revlimit of todays was set...so potentially the bore diameter todays is less than optimum for the revs we see.

A big bore diameter has one real issue - flametravel is a fixed speed event roughly ..so having a big chamber diameter is not what you want as you need time to burn the fuel...

From fia regulations:
ARTICLE 5: ENGINES AND KINETIC ENERGY RECOVERY SYSTEMS
5.3 Engine dimensions:
5.4.1 Cylinder bore diameter may not exceed 98mm
this implies that cylinder bore is flexible

I'd say, 'was flexible', ie prior to engine freeze

[DaveKillens]

5.3 Engine dimensions :
5.3.1 Cylinder bore diameter may not exceed 98mm.
5.3.2 Cylinder spacing must be fixed at 106.5mm (+/- 0.2mm).
5.3.3 The crankshaft centre line must not be less than 58mm above the reference plane.

http://argent.fia.com/web/fia-public.ns ... 3-2012.pdf

Since 5.3.3 limits the lowest limits of height of the heaviest component, the crankshaft, engineers are pretty much forced to set it down at 58mm from the reference plane. Anything higher and the CG (center of gravity) goes up.

Since 5.3.2 forces the distance between bores, you just can't get the cylinders any closer to each other. It would be folly to have a narrower bore (and hence longer stroke) since that too would raise the CG. And if you did have a narrower bore, all that distance between the cylinders would be wasted space, necessitating more mass in the engine. You would have to fill that space with structural webs and channels, extra engine materials.

So the engine designers wind up with a bore of 98mm, something they like in the first place. But as soon as they start to go with less bore, stroke goes up, and as a consequence, mass and CG of the engine rises.

[Tommy Cookers]

suggesting the 'freeze era' rules were written around existing engines (such that more 'advanced' had to make more changes), I guess that the cylinder spacing and crankshaft height were taken from the more conservative (ie Renault)
so that they would need no change to meet those parts of the new rules

and the conservative engines were allowed a redesign to a shorter stroke ?

[olefud]

as far as i understand bore diameter is fixed in the current rules -so there is no leeway for optimising stroke.

The current bore diameter was a function of the revs that were rocketing before the revlimit of todays was set...so potentially the bore diameter todays is less than optimum for the revs we see.

A big bore diameter has one real issue - flametravel is a fixed speed event roughly ..so having a big chamber diameter is not what you want as you need time to burn the fuel...

Flame travel can be enhanced by promoting turbulance in the fuel charge to carry the flame front in an additive fashion to the static molecule to molecule burn. It's not entirely fixed.

[DaveKillens]

(I'm standing on the shoulders of others here)

So although the bore is relatively wide, the flame travel and fuel burn time can be enhanced by a combination of factors.

First is the fuel itself, optimized to allow a burn rate in keeping with the engine design.

Ignition advanced relative to RPM and calculated fuel burn time.

Turbulence is designed in via the geometry of the valves, intakes, and heads to promote a rapid flame front.



A squish area can be created between the piston and head to reduce the area of the cylinder during the initial ignition phase.

The piston pin can be offset from the assumed center line to alter the geometry of the crank/rod/piston geometry.



The ignition itself, although severely limited by the regulations, generates as powerful as possible ignition and heat source.