Rod length to stroke ratio of v6 turbo engines.

[Tommy Cookers]

my point might have been better represented thus .....

we find that recent N/A F1 engines had an unusually large rod ratio (and some unusual reasons for this are apparent)
unusually high bore:stroke ratios seem to be accompanied by unusually high rod ratios
eg Coventry-Climax FPF 94x90 rr=1.44, 92 3.5 V12 Honda 88x48 rr=2.32, 2000 Ferrari V10 96x41.4 rr=2.68
and Cosworth FVA rr=1.77 and DFV 85.7x64.8 rr=2.05
ok rod length seems driven by eg the crankpin and gudgeon pin diameters not being reducable as stroke is reduced
so the rr must rise as b:s ratio rises to these levels
at these high piston accelerations/high rpm/high piston weight the inertia loads become a major contributor to side thrust friction
(the Cosworth CA had max inertia load over 90% of max combustion load, and inertia loads apply on all strokes)
so with very high b:s ratio the inevitable rise in rr (this giving better geometry) gives some relief from this friction

but, typically, don't we 'always' expect a relatively small rod ratio to be better ?
variation of combustion speed is inevitable (and such intermittency is worse without rich mixtures)
given that some heat arrives later than is ideal, the small rr makes equal or better use of this heat (from maybe 30 deg atdc)
this better use may win races (as BRM found)
and the small rr may allow a higher cr


current F1 engines (unlike those N/A) have modest bore:stroke ratio, piston acceleration etc and so should allow freer choice of rr ?
the combustion load is very high, but the pistons might be heavy (eg part-steel 'diesel style' design suggested by Gilles Simon)

[Fede44]

Hi All,

This is my first post here, even though I read you guys daily.

Rod to Stroke ratio has been all around the place since the first GP engines till today, between 1.5 to 2.8

Very long stroke engines would naturally achieve a low R/S, while for short stroke you get easily near 2.8

I would say there is a trend in going back to small numbers for two main reasons:

1) Actual regulations fix the stroke by fixing bore diameter at 80 mm, giving a stroke of around 53 mm to get 1.6 lts. This stroke is a bit longer than previous NA engines, but they took into account that in the hypothetical case someone decides to run (with fuel restriction and current crankshafts) at 15000 rpm, mean piston speed would be 26.5 m/s, which is quite tolerable.

Therefore, the initial engine designer would choose to run a 130 mm con rod to reduce side loads....but the chassis guys would come around and say:

2) We need Reduced weight, reduced packaging => Shorter Con Rod.

The easiest way to achieve a tight packaging around the engine, is to reduce con rod length therefore reducing total engine block height.

So, I think R/S = 2.2 would be ok for current regulations, which gives us a 116 mm con rod.

As usual, even con rods have got influenced by Aero Demands.

[gruntguru]

but, typically, don't we expect a relatively small rod ratio to be better ? given that some heat arrives later than is ideal, the small rr makes equal or better use of this heat (from maybe 30 deg atdc) this better use may win races (as BRM found) and the small rr may allow a higher cr

Agree on the higher cr. Shorter dwell near TDC reduces pressure and temperature of the end gas.

Disagree on energy utilisation. Longer dwell near TDC allows more heat release at small volumes. (The ideal is 100% heat release at TDC.)

[riff_raff]

Hi All,

This is my first post here, even though I read you guys daily.

Rod to Stroke ratio has been all around the place since the first GP engines till today, between 1.5 to 2.8

Very long stroke engines would naturally achieve a low R/S, while for short stroke you get easily near 2.8

I would say there is a trend in going back to small numbers for two main reasons:

1) Actual regulations fix the stroke by fixing bore diameter at 80 mm, giving a stroke of around 53 mm to get 1.6 lts. This stroke is a bit longer than previous NA engines, but they took into account that in the hypothetical case someone decides to run (with fuel restriction and current crankshafts) at 15000 rpm, mean piston speed would be 26.5 m/s, which is quite tolerable.

Therefore, the initial engine designer would choose to run a 130 mm con rod to reduce side loads....but the chassis guys would come around and say:

2) We need Reduced weight, reduced packaging => Shorter Con Rod.

The easiest way to achieve a tight packaging around the engine, is to reduce con rod length therefore reducing total engine block height.

So, I think R/S = 2.2 would be ok for current regulations, which gives us a 116 mm con rod.

As usual, even con rods have got influenced by Aero Demands.

The high rpm N/A F1 engines used as short a conrod length as practical. Provided the best compromise of inertia loads, engine size, performance, etc. The limit of rod length was mostly due to clearance between the piston and crank around BDC. Personally, I think the concerns about conrod length are not warranted. For a race engine, I would use the shortest conrod I could make work.

[Fede44]

The high rpm N/A F1 engines used as short a conrod length as practical. Provided the best compromise of inertia loads, engine size, performance, etc. The limit of rod length was mostly due to clearance between the piston and crank around BDC. Personally, I think the concerns about conrod length are not warranted. For a race engine, I would use the shortest conrod I could make work.

You are mostly correct, and that is why I say that engine designers moved from designing an engine that would outperform on an engine test dyno, to one that would make faster laptimes if helped the car achieving reduced weight and specially packaging.

[strad]

one uses the rod to stroke ratio to move the crank pin as close to 90° as possible at the point of combustion.

[gruntguru]

"point" of combustion?

[riff_raff]

one uses the rod to stroke ratio to move the crank pin as close to 90° as possible at the point of combustion.

What is your definition of "point of combustion"? The combustion process occurring in an SI engine is not instantaneous. Also, is the optimum ~90deg crank pin phasing before or after TDC?

[gruntguru]

The 90* is the angle between rod and crank throw. I don't think the theory has any scientific basis.

[Tommy Cookers]

combustion eg still in race engines today has an element of inconsistency (just as 50 years ago)

eg Flame Speed Measurement of a Racing Engine etc (Ikeda,Nishiyama,Baritaud)
(14th Intnl Symposium on Applications of Laser Techniques to Fluid Mechanics)
shows this in the Ferrari F1 engine c.2005
and references inconsistency elsewhere

this is a reason why some combustion heat arrives later than ideal
and why a smaller rod ratio can work better than might be expected
(and somewhat related to strad's idea ?)

how is combustion speed and consistency in F1 with the presumed very lean, high boost running ?

[strad]

Can ya get any more picky?
What I meant and I think you knew it, was that you want the crankpin to be as close to 90 degrees as you can during the combustion phase.
The reason being that you do not want to waste energy/power pushing any direction other than straight down thru the crankpin.
usually about 1.5:1

[J.A.W.]

Hang on.. "straight" or "90`" ?

90`is a right angle..

& the system is a dynamic recip/rotational/force-inertia-tangent complex..

[Fede44]

Yes, what about those 90 degrees that triggered this conversation?

Would any one be so kind to post some plots so we can put some physics and graphs into the discussion? Looks more like a French engine book (99% words, very accurate though). If not I will, but give me some time.

[strad]

good heavens

[J.A.W.]

good heavens

Strad, that is 'bout on par with your infamous "Quickest or Fastest?" post..

( although "Q or F?" - is actually more technically relevant to the thread topic)..