Mileage

"I thought navy boy's analogy was good and technically correct, IMO. If power is the rate of doing work surely his first example is right?"
The semantic was, that NB said 'the second person makes twice the POWER of the first', but in actual fact that is not true, as the power (torque) has remained a constant, but the work rate (bhp) has increased, I.e. I think he actually meant to say 'the second person can reproduce the POWER twice as fast', which is a different connotation. Otherwise, he put it very well indeed and I'm glad he liked my simple boxing analogy.

I think diesels have a lot of packaging, noise and emissions issues for practical volume use in mainstream motorcycles. It's a shame as I'm quite a fan of turbo-diesel cars, they're generally very real world relevant.

Maybe RRP vs power-to-weight would be a better performance indicator Jag? Ideally an RRP to economy or co-efficient drag-factor ratio would also be useful too?

I wonder how much difference blueprinting or superfinishing/contouring key engine internals would make?

This is a good thread.

I'm facing a choice right now that seems germane to the discussion.

Tomorrow I've got a day's casual work at a stables about 20 miles away. Take the car or the bike?

The car does mpg in the low 40's, gets stuck in traffic, but I can drive it in my work-clothes.

The bike does mpg in the low 50's, nips through traffic, but you have to dress up to ride it. So, I'd have to carry clothes in panniers, then change, and then find somewhere safe for helmet and leathers. They're too expensive to leave lying about.

Seen this way, a bike's not really quicker, and the fuel saving's not that great, is it?

Just been reading through what has been said so far, as quite a few technical bases have been touched! The following appear to contradict each other at first, but thinking it through, I don't believe they do. They are just two different demands. Would appreciate the thoughts of others.

Jag said:    "To get acceptable MPG you generally need high or tall gear ratios or low engine speeds when cruising which would lower the power to weight ratio and performance.

That's why you get better MPG in top gear than when in a lower gear at the same road speed."

Dean15 said:     "I think the bike/car difference we are all missing here is that a big bike runs at small throttle openings nearly all the time. This causes pumping losses in the inlet tract. It also means that the effective compression ratio is much less than the geometric compression ratio."

To satisfy jag's requirement we appear to need to operate at very small throttle openings.

To satisfy dean15's requirement we appear to need to operate at very wide, even full, throttle openings, therefore high rpm and low gears.

How can we satisfy both requirements?

On a related topic. Whilst reading an Internet article on pumping losses, the article suggested variable valve lift instead of using a throttle. But wouldn't this still require the piston to do work to "pull" the air past a small opening? I suppose there would be one less obstacle in the way. Anyone offer any knowledge?

RRP. Recommended retail price! ??

"Nice summary pittsy,"

Thanks.

"You have a challenge before you."

WE do. ; )

"To be honest, good MPG is not a reason why I ride a bike"

Me neither. But lots do and if my commuting needs (job) were different I'd definitely want an economical bike as the solution. When my job was different I used a bike to commute. Usually a 125 or 250 max.

"One possible solution may be to have an infinitely variable transmission."

There's only one type I can think of which isn't grossly inefficient, and that's grossly complicated and expensive.

"I wonder how many would miss the sound of the engine's RPM change as they turn the throttle?"

Me for one. I hate the sensation of variable transmissions. But if it helped solve the particular problem then it would be considered. If we could figure out a way to make them efficient then I can see what you mean.

The device would need to be low powered, low revving and variable displacement. Basically something like an old panther with variable stroke. Hee Hee. But a lot lighter! Now that's what I call a contraption.

"A quarter of a Jazz"
... I like your thinking Pitts! Do you think it'd get close to 100 mpg, or would the overall weight hamper it too much to achieve that?

Dean, you're making me want to hang up the helmet! ;-D

RRP is Recommended Retail Price (out the door price)

"One possible solution may be to have an infinitely variable transmission"
... I wonder what the economy differences are between the manual and DCT shift VFR?

"Surely the weight would go down proportionally with the power rating and top speed potential"
... weight would go down, but so would power. I.e. PTW ration will drop. So although the slightly lighter weight and less cylinders has the potential to be more frugal, the engine will also have to work harder off-setting the benefit. The upshot is I don';t know if mpg would improve or be worse! :-D

Hi Pittsy,

I would assume you'd want the bike to run on regular gas. Here regular gas is 87 octane. A water cooled 4 valve head would help but the lower octane fuel would limit the compression ratio.

Looks like you are talking about a 250 cc single cyliner engine.

A nice stroke would be 66 mm. This would give you a piston speed at maximum RPM of 3900 ft/min at 9000 RPM.

To get 250 cc displacement the bore of the single cylinder would be 69.5 mm.

The total piston area would be 5.877 square inches.

At 4.25 RWHP per square inch of total piston area at 9000 RPM (3900 ft/min piston speed) you might get up to 25 RWHP.

At 20 crank HP I think you'd be ok under 100 Km/h but there might be power issues at higher highway cruising speeds (110 to 120 Km/hr) in terms of power reserve for acceleration and head winds and hills.

I don't know if this engine would get you your 100 MPG.

Regards,

JAG

In fact our regular unleaded is 95 Ron.

Jag said: "Engines don’t suck."

I can see what you mean. So, aside from the frictional and inertial losses, the main work the piston is doing, at this point, is working against the higher pressure beneath it. It's as if the piston is trying to push down with effort equivalent to compressing a gas of a pressure equal to the difference. I can see how the partial vacuum will increase with revs, for the same throttle position. I'm struggling to see how the energy will be recovered at bdc though. As I understand it the inlet valve remains open past bdc and gas continues to fill the chamber. So wouldn't pressure in the chamber very quickly build up, overwhelming the nominally atmospheric pressure beneath the piston in only a few degrees of rotation? Robbing energy built up as inertia or from other cylinders firing, if there are any, in having to compress the new gas on the way to tdc?

If I've got this right, the whole thing with a throttle is to do with volumetric efficiency. To deliberately prevent good cylinder filling. The problem surely then, is that the amount of gas needed for the job, at that particular point in time, is mismatched with the cylinder volume. In other words the engine is too big. Any time you're not using full throttle your engine is too big for the job! The answer then is variable capacity, and the only way I can think of to get this is by varying the stroke. Or by cutting cylinders if there are more tan one. The more cylinders involved, the smoother the transition and the more accurate the match of volumetric efficiency for the job.

With variable stroke the problem, apart from extra weight and friction (and cost) is maintaining the correct compression ratio all the while.

Have I got any of that right?

Quite a task you've got there!

Erm, I don't think so

http://www.fiat.co.uk/Content/Default.aspx?id=3504

Fiat's multijet appears to be Diesel only, and will not have a throttle valve at all.

I've been wondering if a DI petrol engine could be built with a very high geometric comp ratio. In normal running it would shed some of this compression by briefly opening the exhaust valve during the compression stroke, then reseating it. When economical cruise is required, open throttle fully; limited supply of diesel fuel to injectors; full compression ratio. It's a petrol engine that turns into a diesel for cruising.

Some engines like the new Ford 3 cylinder don't have cams; they use electro-hydraulic actuation. That means you can do anything with the valve timing.

In doing away with the "throttle" butterfly and using variable valve lift are we not just using the inlet (poppet) valve(s) as a.throttle instead? What's the difference?

I didn't realise that someone had come up with electro hydraulic actuation. I'd always assumed that, although the idea is obvious, it wouldn't be suitable for high ish engine speeds.

If we are looking for a way of altering the geometric compression ratio downwards then perhaps a swashplate/wobble plate engine is not so daft after all.

Pittsy's swash plate variable displacement engine.

I suspect you are comparing a variable displacement hydraulic piston pump or motor with piston shoes.

These piston shoes are hydrostatically balanced which means a small amount of high pressure oil from the piston is leaked through a small hole to the bottom of the piston shoes between the shoe and swash plate or cam. This film of oil prevents the shoes from contacting the swash plate. That is why the shoes and swash plate are not damaged at high pressures.

This feature was actually designed many years ago by the company I worked for Denison.

This would be hard to do with a gasoline engine.

Besides, all gasoline engines are variable compression pressure engines anyway so in a sense act like a variable displacement engine.

Regards,

JAG

Of course one far simpler way around this would be to utilise the engine in a different manner. That is design engines that are at, or very close to, their torque peak at real world speeds when in top gear.

I'm willing to bet that the Honda NC bikes are close to full torque at real world speeds (60-80 Mph) when in top gear. My Alfa diesel makes its maximum torque around 2000Rpm. In 6th gear it is doing around 70 Mph at this engine speed. Not surprisingly it returns its best fuel figres when one sits at 65-70 Mph.

I'm willing to bet that my old Triumph TT600 would have actually returned better fuel figures (In normal cruising anyway) had I lowered the gearing a little.

Single track urban transporters

I was reading Cycle World’s road tests on the Harley Davidson Softtail Slim and the BMW C650GT “Urban Mobility Vehicle” and it occurred to me how much they have in common. Performance and weights are comparable. Both are designed for the same environment. Their approaches are visually very different.

I find it interesting that BMW calls the C650GT an “Urban Mobility Vehicle”. One gets the impression they almost want to distance the model as far from motorcycles as possible. Much like Honda did in the early days when if a prospective buyer wanted to by a motorcycle from them the sales person would say they would sell you a Honda, not a motorcycle.

I am surprised just how heavy these urban movers are. The Burgman is well over 600 lbs wet. Even the BMW C650GT comes in at about 580 lbs wet.

For Pittsy’s 100 MPG bike to achieve this high level of efficiency, weight becomes a major factor.

For our 250 cc bike to achieve a reasonable performance level a total weight of under 325 lbs. should be the target.

I envision an enduro type of bike with street tires and good lighting with a reasonably sized tank for over a 150 mile range before the reserve comes on.

The engine should have a flat torque curve for maximum flexibility. Maximum torque would be at about 7000 RPM but still able to achieve 90% of that at 4000 RPM.

Suppose the maximum speed was 80 MPH at 9000 RPM in top gear and maximum torque was at 7000 RPM.

In top gear 7000 RPM (maximum torque) would equal 62.2 MPH and 4000 RPM would equal 35.6 MPH.

For the real hot roders there is always the possibility of a nice 450 cc single.

I have always wanted to try a good 450 or 500 cc single cylinder engine in a light package with good brakes and handling.

Regards,

JAG

Captain scarlet said: "Maybe what we're looking for, mileage wise, is something betwixt a Yamaha SDR200 (the 'whippet') and Honda XBR500, wearing a dustbin fairing, resplendent in RC166 colours, ......."

Yes. Yes.

Then...

".........ridden by a CSS trained capuchin monkey?"

Doh!

You were going so well.