22 replies to this topic

[Manson]

Ok, the title is very vague but I couldn't think of anything else.

The majority of the engines in the Club are 91-95 Kawasaki ZX7 motorcycle engines. The stock bore and stroke is 71 mm X 47.3 mm. We are allowed to go 1 mm over stock on the bore (72 mm). The 96-2003 model has a bore and stroke of 73 mm X 44.7 mm.

The rules allow the 91-95 models to be stock appearing with a maximum displacement of 770 cc. The 96-2003 model has to be stock, period, with a maximum displacement of 750 cc. The older model engines have to run at 900 lb. car and driver vs. 870 for the later model engines. The 91-95 engines are still quicker (no doubt) but the stock engines have better reliability (for me at least).

Since the shorter stroke crankshaft will fit in the 91-95 engine, I can build an early style motor with a stroke of 44.3 mm and with the 1 mm over allowance, I can run 74 mm pistons for a displacement of 770 cc.

What advantages does a short stroke motor have over a longer stroke? From personal experience, it seems the shorter stroke motor has a narrower power band and less bottom end than the longer stroke motor but runs well at constantly high RPMs. We run a couple tracks where the RPM change over the lap is minimal. I think a short stroke motor built with cam timing to match power at the correct RPM would be kick ass at these tracks.

thoughts?

[VAR1016]

I am posting a reply principally because I am very interested in this and also the significance of the con-rod to stroke ratio and I know that there are some very clever people hereabouts from whom I hope to learn.

A shorter stroke will mean a lower effective piston speed which means less liklihood of piston ring flutter at high revs, the price being (I think) greater angular thrust - is there perhaps a friction penalty here? On the other hand, traditionally power has been related to piston area so for a given cubic capacity, a short stroke means more piston area.

In his book "It was fun" Tony Rudd relates how he discovered, when the 1.5 litre V-8 was enlarged, that the engines with a proportionately shorter rod gave greater specific horsepower.

I look forward to reading the views of the experts (of which I am not one!)

PdeRL

[Powersteer]

Manson..I am no expert but here goes.


Old engine will get you an additional 20cc. At 100bhp/liter thats an additional 2 horsepower or 4 horsepower if it was 200bhp/liter. The penalty for this is 30ib. The longer stroke engine will give you the torque which i don't know how much.

With new engine you probably get an engine that has an improved design which might make up for the 2 - 4 horsepower the extra 20cc advantage the old engine has. In addition you have 30ib less weight which might be the option Colin Chapman would take But from the way i see it, it can be more expensive with the new engine. By the way, what are the difference in power between the old engine and the new one at stock 750cc?

[Manson]

Originally posted by Powersteer
By the way, what are the difference in power between the old engine and the new one at stock 750cc?

Hmmm...tough to say. I hear that the new motor makes 124 hp stock and that some of the tweaked old style motors make 135-140 hp. The older motors are more expensive to build as you can run aftermarket rods, pistons, cams, lightened cranks, springs, you name it. The Club wants to move toward the stock option for costs savings but 30 lb. to make up the hp difference probably won't cut it. Most of the owners use the older style engines and since the owners make the rules, it was hard enough to go from 15 to 30 lb. let alone give the new engines any other advantages. Hence, I can build both with what I have with complete motors and engine parts. The idea of building the stroker is thinking out loud for the moment.

[McGuire]

This response is somewhat OT as it relates to engine design rather than development (which is more your question) but here are a few general observations: 

The primary advantage of a higher bore/stroke ratio is topographical: a larger cylinder diameter allows more valve area per unit of displacement.

The second advantage is geometrical. The longer the stroke, the further the piston must travel per crank rotation, so a shorter stroke reduces piston speed relative to crank speed. However, modern metallurgy and processes have made piston speed far less a factor in engine durability than it once was. Back in the day, the limits of the technology imposed an unofficial speed limit of 4000 fpm, which is now pretty much out the window.

It's a myth that a long stroke engine will make more torque than a short stroke engine of equal displacement. It simply tends to make its peak torque at a lower rpm -- mostly due to its smaller valve area, which produces high port velocities at lower rpm (but runs out of flow volume at high rpm). So given a long stroke design, it will be optimized for the rpm range to which it is suited: toward low-to-mid range torque. When looking over engines for possible racing development, bore/stroke ratio is a pretty decent sign of the rpm capabilities in the rest of its design.

But all things being equal (including valve area etc) a large bore/short stroke engine will make both MORE brake torque and brake horsepower throughout the rpm range.  Within the historical range of bore/stroke ratios, the difference in peak bhp may be up to 5%. Essentially, a longer stroke absorbs more friction hp in ring drag. Also, the last 10-15% or so of the stroke is relatively inefficient, so the longer the stroke for a given displacement the greater the resultant loss there.  Most engine sims will illustrate this: "build" two engines of identical displacement but with bore/stroke ratios at each extreme, and it will prove out.

[VAR1016]

Originally posted by McGuire
This response is somewhat OT as it relates to engine design rather than development (which is more your question) but here are a few general observations: 

The primary advantage of a higher bore/stroke ratio is topographical: a larger cylinder diameter allows more valve area per unit of displacement.

The second advantage is geometrical. The longer the stroke, the further the piston must travel per crank rotation, so a shorter stroke reduces piston speed relative to crank speed. However, modern metallurgy and processes have made piston speed far less a factor in engine durability than it once was. Back in the day, the limits of the technology imposed an unofficial speed limit of 4000 fpm, which is now pretty much out the window.

It's a myth that a long stroke engine will make more torque than a short stroke engine of equal displacement. It simply tends to make its peak torque at a lower rpm -- mostly due to its smaller valve area, which produces high port velocities at lower rpm (but runs out of flow volume at high rpm). So given a long stroke design, it will be optimized for the rpm range to which it is suited: toward low-to-mid range torque. When looking over engines for possible racing development, bore/stroke ratio is a pretty decent sign of the rpm capabilities in the rest of its design.

But all things being equal (including valve area etc) a large bore/short stroke engine will make both MORE brake torque and brake horsepower throughout the rpm range.  Within the historical range of bore/stroke ratios, the difference in peak bhp may be up to 5%. Essentially, a longer stroke absorbs more friction hp in ring drag. Also, the last 10-15% or so of the stroke is relatively inefficient, so the longer the stroke for a given displacement the greater the resultant loss there.  Most engine sims will illustrate this: "build" two engines of identical displacement but with bore/stroke ratios at each extreme, and it will prove out.

A very concise summary if I may say so

Would you care to share your understanding of the significance of con-rod to stroke ratios?

Thanks

PdeRL

[LandOfSnow]

I believe that in high compression solutions extreme bore/stroke (big bore) might not be ideal 'cause the shape of the burn chamber is too low and wide. (?) By "extreme" I'm thinking something like 1.7-2.0 / 1 .

[Bill Sherwood]

Nah, big & carefully shaped squish areas are good things for best power.

[J. Edlund]

While a larger bore tends to increase power it also decreases efficiency and increase exhaust emissions, those are however usually less important for racing purposes.

[VAR1016]

Originally posted by J. Edlund
While a larger bore tends to increase power it also decreases efficiency and increase exhaust emissions, those are however usually less important for racing purposes.

How can this be?

If it is agreed that for a given capacity proportionally larger bore will give greater torque and horsepower, then volumetric efficiency must surely be greater?

Of course since greater power and torque are being produced one would expect increased fuel consumption, but why the extra emissions - and which emissions? CO2 of course because more fuel must be consumed. For maximum power CO will be about 6%, but this is universal, so is it NOx?

Best wishes

PdeRL

[LandOfSnow]

IMO the reason the efficiency decreases and pollution increases because of the poor shape of the chamber. Long distance and narrow flame front. Especially in high C/R engines where the chamber has to be very low to achieve the C/R.

[Powersteer]

Long stroke gives a mechanical advantage of having longer leverage on the crank shaft with the same cubic capacity giving more kick to the torque like bicycles long crank.

[VAR1016]

Originally posted by LandOfSnow
IMO the reason the efficiency decreases and pollution increases because of the poor shape of the chamber. Long distance and narrow flame front. Especially in high C/R engines where the chamber has to be very low to achieve the C/R.

Well the flame path issue has traditionally been solved by the use of dual ignition; and again if the engine produces higher specific BHP and torque it must be more efficient..

I would like to know if there are corollaries here: e.g. if volumetric efficiency is good, leading to good B.M.E.P. then the other parameters one might look at are thermal efficiency and fuel efficiency. I have always assumed that these must be tied together, but am I correct in thinking this?

PdeRL

[LandOfSnow]

The engine can produce more torque even if the burn is less efficient. You just need more air & fuel in the chamber (valve area). The loss of burn efficiency is made up by the increase in the fuel to be burned. Question is how low/wide can the chamber be made until the extra breathing capacity doesn't anymore make up the efficiency loss. This also depends on the other parameters of the engine, ie. is the valvearea the limiting factor? If not, there's not much to gain by the increase.

Dual ignition is quite rare and difficult to add to a watercooled cylinderhead aftermarket. Can be done, but difficult. CCC (Walker) built a ford Ohc with a dual ignition head as they believed they had reached the limit of the flame front's speed and couldn't increase the power with extra revs/flow.

[VAR1016]

Originally posted by LandOfSnow
The engine can produce more torque even if the burn is less efficient. You just need more air & fuel in the chamber (valve area). The loss of burn efficiency is made up by the increase in the fuel to be burned. Question is how low/wide can the chamber be made until the extra breathing capacity doesn't anymore make up the efficiency loss. This also depends on the other parameters of the engine, ie. is the valvearea the limiting factor? If not, there's not much to gain by the increase.

Dual ignition is quite rare and difficult to add to a watercooled cylinderhead aftermarket. Can be done, but difficult. CCC (Walker) built a ford Ohc with a dual ignition head as they believed they had reached the limit of the flame front's speed and couldn't increase the power with extra revs/flow.

OK, of course I meant dual ignition as with the Alfas and the Aston Martin DB4GT.

911 Porsche engines always had an extra boss to allow for dual ignition, but few could afford to buy the 12-plug distributor!

PdeRL

[LandOfSnow]

Air-cooled (performance) engines often have the possibility to fit an extra plug, old singe-cyl. Ducatis and 911 for ex. It's easy to fit into air-cooled desings.

[McGuire]

Originally posted by LandOfSnow
I believe that in high compression solutions extreme bore/stroke (big bore) might not be ideal 'cause the shape of the burn chamber is too low and wide. (?) By "extreme" I'm thinking something like 1.7-2.0 / 1 .

They seem to have blown right past the 2:1 benchmark in contemporary F1 practice. For example the BMW P82 (reportedly) had a bore of 98mm and a stroke of 39.75mm.

[desmo]

While F1 engines with their rather unique set of compromises forced by the
technical regulations make the particulars of this paper published in 1998 likely less than fully applicable to other types of engines- particularly the compromises in combustion chamber geometries forced by the extreme valve lifts (18mm+ likely), still I'm sure some of the conclusions
reached by the paper's authors apply more generally.


The following excerpts are from SAE TECHNICAL PAPER 980126- The Influence of Stroke-to-Bore Ratio and Combustion Chamber Design on Formula One Engines Performance, available from the SAE.

"...within real life design constraints. The effects of the
stroke-to-bore ratio on both the volumetric efficiency and
the thermal conversion efficiency have been investigated.
Flame front area maps, wall areas wetted by burned
gases, mean flow field patterns and main turbulent
parameters have been compared for two different S/B
ratios. Since higher intake and exhaust valve areas per
unit displaced volume result in a higher volume of piston
bowls, a lower S/B ratio leads to a lower compression
ratio, which strongly limits the indicated mean effective
pressure. Therefore, in the second part of the paper, an
analysis of the influence of the piston shape on combustion
process has been performed in order to optimize the
imep of the lower S/B ratio engine.

The overall technical objective of racing engine design is
the achievement of the highest possible power levels. A
simple analysis can reveal the effects of the main engine
parameters on the brake power Pb delivered by a fourstroke
cycle reciprocating internal combustion engine

For a given total displaced volume, high engine speed, high compression
ratio, high volumetric efficiency and low stroke-to-bore
ratio are required in order to obtain high levels of power.
High rated engine speeds dictate a diminishing of the
stroke due to mechanical stress and friction considerations.
High volumetric efficiencies call for higher valve
areas per unit displaced volume, resulting in a bore that
increases with the rated engine speed.
On the other hand, the diminishing of the S/B ratio determines
a lower thermal conversion efficiency since it leads
to a combustion chamber design far from the optimum
one required for an efficient combustion process.

In fact,as the S/B decreases, combustion duration increases, as
well as cycle-by-cycle variability and chances of misfire
or partial burning.
Since performance improvements call for low stroke-to-bore
ratios, a detailed study of in-cylinder processes is
required in order to achieve a compromise between thermal
conversion efficiency and volumetric efficiency.

Since higher intake and exhaust valve areas per unit displaced
volume resulted in higher volume of the piston
bowls, the diminishing of the S/B ratio led to a lower compression
ratio. Although this is often sufficient for avoiding
knock, it strongly limits the imep. Therefore, in the second
part of the paper, an analysis of the piston shape influence
on combustion process has been performed in
order to optimize the compression ratio of the lower S/B
ratio engine.

The objective of a combustion chamber is to produce
fast, stable and repeatable combustion processes with a
high thermal conversion efficiency. The objective of racing
engines is to provide a high specific power by achieving
a high thermal conversion and high volumetric
efficiencies.
For a fixed total displaced volume, high power density
requires high rated engine speeds. Hence, to obtain high
volumetric efficiencies at high engine speeds and to limit
friction and component mechanical stress, low S/B ratios
must be adopted.

Due to the higher maximum speed and volumetric
efficiency, for constant fuel conversion efficiency, the
engine with higher bore to stroke ratio should have about
a 5% gain on brake specific power. The poorer engine
fuel conversion efficiency is expected to reduce this gain.

A CFD analysis was carried out in order to gain more
insight regarding the effects of S/B ratio changes on racing
engine performance. Two different S/B ratio engines
were compared: since the reduction of S/B ratio was performed
within actual design constraints, the low S/B ratio
engine presented a 3.2% lower compression ratio. The
analysis of the influence of S/B ratio on volumetric efficiency
and thermal conversion efficiency led to the following
conclusions:

• Decreasing the S/B ratio results in the improvement
(up to 4%) of the volumetric efficiency in the range of
medium to high engine speed, while it determines a
reduction in the volumetric efficiency at low speeds.

• The thermal conversion efficiency falls while S/B has
decreased as a consequence of the much lower
burning rate due to both the lower compression ratio
and the lower fuel mass effectiveness. The reduction
of imep is balanced by the increase in the maximum
rated power rotational speed. However, as a result,
the two different S/B ratio engines present the same
indicated power.

• In order to achieve the highest possible gain from a
S/B reduction, an optimization of the combustion
chamber, with particular emphasis on compression
ratio, is required.

• Calculations clearly show the important role played
by compression ratio on engine performance. An
increase of the compression ratio of 3.2% determines
a power gain of about 2.9%.

• Finally, computations reveal that a proper optimization
of the piston shape could produce significant
improvement in the combustion process quality."

[VAR1016]

Thanks, Desmo for reproducing that very interesting extract.

Now, does the paper deal with the effects of rod/stroke ratio?

Thanks

PdeRL

[Yelnats]

[QUOTE]Originally posted by Powersteer
Long stroke gives a mechanical advantage of having longer leverage on the crank shaft with the same cubic capacity giving more kick to the torque like bicycles long crank.


[/QUOTE

And exactly compensating for this is the extra kick provided by the larger piston area of the short stroke engine. "There is no free lunch"

The issue of which engine (short or long stroke) produces the most tourqe lies elsewhere than the stroke length and centers on combustion chamber design. If the stroke is sufficiently short (say 1.5 times the bore) the initial chamber shape will be excessivly thin and wide with deep valve scallops which will limit compression ratios and produce inefficiencies in fuel burning due to excessive cooled surface area and greater heat losses during the early combustion period.

This COULD limit tourque during low rev operation but carefull design will limit these losses in the conservative S/B ratios used on road engines.

[McGuire]

[QUOTE]Originally posted by Yelnats
[QUOTE]Originally posted by Powersteer
Long stroke gives a mechanical advantage of having longer leverage on the crank shaft with the same cubic capacity giving more kick to the torque like bicycles long crank.


[/QUOTE

And exactly compensating for this is the extra kick provided by the larger piston area of the short stroke engine. "There is no free lunch"

[/QUOTE]


In depression-era USA some bright sparks hustled something called the "Powell Lever Engine" around the county fair and trade show circuits. It was a conventional L-head six more or less, except the connecting rods communicated with a second motion linkage, thence to the crankshaft. They claimed more torque for their engine, which of course was exactly true. What it accomplished beyond simply lowering the final drive gear was never explained...or understood, possibly.

[Powersteer]

Yelnats, I was merely refering to one area of torque a long stroke can contribute. To think that torque can be generate through this one area is absurb as you and i know they are many area to do it. By the way, i cant see big bore giving any mechanical torque advantage besides having less friction loss, a long stroke engine also would provide smaller and longer cylinder wall that would give a longer and more concerntrated push to the piston, like having a large cannon shooting through a rifle. Again, for the same BMEP a long stroke has that mechanical advantage, MECHANICAL. BMEP competence is another thing.

[Yelnats]

Originally posted by McGuire



In depression-era USA some bright sparks hustled something called the "Powell Lever Engine" around the county fair and trade show circuits. It was a conventional L-head six more or less, except the connecting rods communicated with a second motion linkage, thence to the crankshaft. They claimed more torque for their engine, which of course was exactly true. What it accomplished beyond simply lowering the final drive gear was never explained...or understood, possibly.

Yes I have heard of this scam but can't recall the details of the design. These type of ripoffs are ever with us, consider the inlet "turbo" fan scam heavily promoted on TV in the USA this fall.