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 Whats in a number?

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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Whats in a number?   Sun Dec 13, 2015 7:29 am

Comparative hp/ltr, bmep and piston speeds of 50s and Bantams
Rear wheel hp is converted to higher crank power by a factor of 1.13
BMEP is average cylinder pressure through one engine cycle at peak torque rpm.

13hp – 50cc (1000/50) X 14.7 = 294 hp/ltr. 13,500 rpm 9.63 bar bmep. 17.8 mtrs/sec. 39.5mm stroke
22hp - 125cc (1000/125) x 24.9 =199.2 hp/ltr. 11,000 rpm 8.0 bar bmep. 19.8 mtrs/sec. 54.0mm stroke
25hp –186cc (1000/186) x 28.2 = 153.2 hp/ltr. 8,250 rpm 8.15 bar bmep. 15.9 mtrs/sec. 58.0mm stroke

During the various conversations with the visiting 50cc experts to the Avenue on the 29th, some of the above data was suggested for the performance of their engines!
Considering the tiny 40mm bore of a typical 50cc race engine and the added physical complications of relative scale, that figure of 294hp/ltr is really remarkable. So Bantam men still have a long way to go to emulate the achievements of the 50cc tuners! Quite humbling really!
Reflecting back to the 1970s when the GP disc-valve 50s were churning out over 21hp, and then fast-forward and incorporating today`s technical advances into those engines, 25hp would be readily achievable. Oh my, we still have much more to do to openly stand alongside the best engineers of those wonderful little engines.
Isn`t that the category where Jan Thiel started his illustrious career?

BMEP potential and common piston speeds are very useful guides to possible torque values and engine stresses respectively, enabling comparisons to be made between engines of differing capacities. The figures shown here are reported hp and rpm figures offered in the context of Bantam racing and attendant categories and I`m sure we are all familiar with them. Notwithstanding that, the numbers are both revealing and informative.
Intriguingly, the 186cc bantam engine is mechanically, comfortably under stressed, and developing the tuning envelope a little higher, pushing the revs up to, say, 9,000 at peak torque will make it very hard to beat. Accordingly it would be prudent to pay attention to getting rid of the extra thermal loading from that big bore air cooled cylinder by the intense purging of hot air trapped between closely spaced fins where little moving air actually reaches!

From the mid-70s to the 1990s average bmep for two-stroke race engines increased by some 60% whilst piston speeds remained virtually constant, and in its final years the 500cc Honda NSR managed 365hp/ltr. The Aprilia 125 engine of 2009, its final year of development, stands alone producing a massive 16 bar BMEP, double that of the current crop of 125 Bantams, but at only 18% higher revs. With 480 hp/ltr this engine remains the exemplar against which all other are judged, food for thought!

With 60 crank hp available for the 125cc Aprilia, an interesting and surprising comparison can be made with another 60+ hp single cylinder engine, that of the 500cc QUB. As shown earlier the Aprilia generates 16 bar at 13,000rpm but the big QUB has only 7.15bar at the lowly rpm figure of 7,500 and a hp/ltr figure of just 122 from its 76mm stroke length that in turn produces a high mean piston speed of 19 mtrs/sec, the 54.5 stroke of the Aprilia at 13,000 rpm, is revving nicely at 23.6mtrs/sec!
Looking at the case-comp ratio numbers for these two engines throws up some startling sums, in the case of the Aprilia at 1.23:1 and for the QUB, 1.65:1. Using the following equation;
TDC volume = comp-ratio / (comp ratio-1) x cylinder volume
The volumes are revealed as 675cc and 1254cc, a huge disparity in volumes but producing similar power outputs?

Just a load of numbers perhaps, but with some commonality between vastly different engine capacities, however, they do tell a quite complex story, but none the less for that, a highly intriguing one?

Trevor

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john bass

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PostSubject: Something I was going to ask...?   Wed Dec 16, 2015 5:49 am

Must have read my mind, Trevor -- I was about to ask what the primary crannkcase compression ratio ought to be on the 125 Bnatm and what about the transfer port volumes: in practical terms could they be too large ,in relation to the crankcase volume?

Cheers!

JayBee...
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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Re: Whats in a number?   Thu Dec 17, 2015 6:56 am

Back in our day John, we went as high as was possible with case pressure within the means and knowledge available to us. One of George`s horse shoe rings and some ally sheet was all! Mind you the old iron barrel transfer duct did a pretty good, bearing in mind the limited demands placed upon then with most long stroke engines achieving around 5bar!
The requirements of even an 8+bar Bantam engine are very different to a 12+bar reed-valve, pukka race engine that has all of the ancillary design requirements that contribute to that level of output.
The Aprilia engine needs a huge case volume and breathes through a 42mm carb at 13,000rpm to make 60 crank hp with an effective base to it`s power range starting at 10,000 rpm. No Bantam can replicate that specification and so cannot produce that level of performance, and as such a big case volume would be not appropriate!
For some time now the trend has been reduce the volume of all engine ducts. The idea is to reduce the inertia of the initial, static volume available that has to be accelerated out of the transfer duct into the cylinder, and that requires a certain time lapse to achieve.
The volume of available mixture is regarded as being: specific mass x duct length x cross sectional area.
To move mixture in a certain time frame necessitates a certain flow velocity, and that velocity is inversely proportional to the cross area. And, the pressure difference between case/duct and cylinder is proportional to the square of that velocity. When the pressure differential between case and cylinder equalise the flow velocity is at its maximum! Quite simply stated, big and /or long ducts slow down the mass transfer of mixture to the cylinder. That maybe ok at peak revs but is not good for low speed.
At low engine revs, just before the pipe starts to work, the engine will respond to pumping from a case with minimum dead volume. Gradually mixture movement come under the influence of the exhaust pipe`s diffuser which creates the pressure differential that provides for mixture movement.
However, as revs rise to peak at, say 11.000+ rpm, there may be insufficient time available for the pipe to suck sufficient mixture through the engine to fully empty the case and transfer ducts. In these situations the cylinder may only receive that mixture that in in the transfer duct, and don`t take it for granted that those ducts and crank case was full in the first place! If you were able to calculate the amount of transferred mixture, ie the delivery ratio, it might be found that the delivered quantity is less than the potential volume of the ducts?
8bar is a good representative number for a Bantam race engine output but is modest compared to the standard of pukka 125 race engines
Taking an overview of case volumes and taking out the non-typical Aprilia average case compression ratios range between 1.3-1.4:1 .Big case volumes bring problems in their wake, and Bantams have too few gears to cope with the consequences of them. To get the best from down at 1.3 you need free-flowing transfer ducts of short length and large cross areas, and a big carb to feed it. The larger volume has a lower natural resonance that can mess with fueling signals to the carb and reed frequencies, if you have reed induction.
So for low power Bantams being up around 1.4:1 does not restrict mass flow but does widen the effective power range. All of this pre-supposes that the secondary comp ratio is averagely high, the ignition retards at the appropriate revs, the duct profiles are reasonable and most important of all is that the pipe works at the important points in the crank rotation.
At all times it has to be remembered that these numbers are ratios, non-dimensional figures, and make no reference to the actual quantity of mixture contained within the volume in question, and it is the mixture that makes the power!
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john bass

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PostSubject: Thanks Trevor...    Sat Dec 19, 2015 5:22 am

Thanks Trevor!

Sorted out a little of what´s between my ears....

Cheers!
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