Why do bikes rev higher than cars?

[Stan 24v 0]

And why do petrols rev higher than diesels, etc etc. :?

[dr_mat 0]

Big question. Some points:

 

Bike engines are small, have low mass pistons, less inertia in the metal parts that are oscillating. Also they simply don't need to generate big torque, so they don't need long throw or overly strong cranks. They just need power, which is easiest obtained by spinning it fast.

 

Petrol engines have the ability to control ignition timing, which allows them to fire the spark well before the piston actually reaches TDC. This means that the (comparatively slow) flame front will meet the piston crown *as* it reaches TDC. Can't do this on a diesel, you have to wait for TDC (or thereabouts) before ignition takes place, and so at very high revs the piston is already moving away from the flame front before it can get there and actually PUSH the piston down, thereby wasting the energy, if you see what I mean ..?

 

They are working on increasing the rev limits on diesels by doing clever things with the injectors, getting it to ignite before TDC to improve matters. But let's face it diesels were designed to be extremely efficient running at a single RPM, to be used for generators, boats, etc etc. The fundamental design is very good, but it's being applied to something it's not suited to. Whack a continuously variable transmission on one though, and it'd be perfect. (Whatever happened to them??)

[ZippyVR6 0]

Whack a continuously variable transmission on one though, and it'd be perfect. (Whatever happened to them??)

 

 

Like on outboard boat engines?

[chubbybrown 0]

I think the Dr means CVT usuaually rubber belt transmission.

Should have caught on but didnt.

[Toad 0]

There's a pretty big thing in agriculture with cvt boxes at the moment, using hydraulic pumps, motors and planetary gearing, most do have between 2 and 4 'gears' these are all controlled by the computers so you shouldn't know about them when you're driving.

The idea is that the engine management and gearbox computers talk to each other to get the best efficiency or power etc for particular tasks. It does mean that a 10 tonne tractor can hit 40k in about 6 seconds too. which is shifting a bit.

I did manage to get a big fendt with CVT sideways on a roundabout by hitting the go faster button when i shouldn't have done, the big dung spreader pushed the back of the tractor round, I very nearly soiled myself, had to take to the grass verge to get myself out of trouble.

[dr_mat 0]

Yeah, "rubber belt" type, was what I was referring to. Though they tend to use steel belts to transmit the torque properly.

[ZippyVR6 0]

I did manage to get a big fendt with CVT sideways on a roundabout by hitting the go faster button when i shouldn't have done, the big dung spreader pushed the back of the tractor round, I very nearly soiled myself, had to take to the grass verge to get myself out of trouble.

 

Nearly soiled yourself and the rest of the road:)

[Toad 0]

I did manage to get a big fendt with CVT sideways on a roundabout by hitting the go faster button when i shouldn't have done, the big dung spreader pushed the back of the tractor round, I very nearly soiled myself, had to take to the grass verge to get myself out of trouble.

 

Nearly soiled yourself and the rest of the road:)

 

Haha very nearly mate. Still not as bad as the time i intruded into the back of a transit with a tractor! :oops:

 

Sorry, bit off topic, I know.

 

 

You do get some fairly big capacity bike engines nowadays don't you? surely they must have more mass in the pistons/conrod/crank which would limit the revs?

 

I guess the real high revving engines would be on four strokes etc, or can two strokes reach high rpms? My bike kniowledge is limited to the CR80 i had as a child.

[dr_mat 0]

Even the big bikes don't use such long conrods and such eccentric cranks as car engines because there's simply no need for big torque.

[Kevin Bacon 5]

Yep, Dr is correct. Low rotating mass, inertia and friction are what gives you stratospheric rev ranges. The current F1 V8s rev to about 20K (as high, if not higher than a lot of bike engines), which is achieved by ultra low internal friction (tiny piston skirts), ultra light and thin conrods, over square bore/stroke and special cranks, very wild cam profiles (idle speed is 5K), high compression ratio, clever timing etc etc.... you name it, it's all other world exotica. Million quid an engine.....used to be rebuilt every race but new rules state the engine has to last a season now IIRC? F1 cranks cost ~ £150,000 a time and take 8 weeks to make by hand.

 

If a petrol engine didn't have a throttle size limitation or a rev limiter, it would keep revving and revving until something broke or ran out of fuel flow, so high revving engine producers exploit that fact but make sure everything is light and strong enough to sustain continuous high revs. It's usually when the conrod starts it's down stroke that causes problems. The crank puts a huge whiplash kind of force on the rod..... and the bolts, conrods etc have to be strong enough to withstand it. And at those kind of revs, cam timing becomes critical too...but the both inlet and exhaust valves are almost permanently open!

 

In the case of diesels, they're made for torque and torque comes from leverage, so that means high compression and long throw cranks. Due to the internal forces Diesels have to endure (fuel explodes rather than burns), they have to be beefed up and beefiness means weight. Long throw and weight = not a big rev range. Diesels used to need something like 24:1 CR, but they've got it down to about 15/18:1 now by using a turbo which has increased the rev range. When/if they make diesel blocks with lighter alloys and get smart with the fuelling, then they'll rev like petrols......but if they do, you may aswell stick with a petrol as it'll quieter and smoother and the diesel's torque advantage would be lost if they keep reducing the CR.

 

DISCLAIMER - That is a culmination of what I've read, tried and remember from my college days ;-)

[davidwort 0]

... Whack a continuously variable transmission on one though, and it'd be perfect. (Whatever happened to them??)

 

That'd sound great, an oil burner constantly rattling at 3000 revs all the way down the road, it was bad enough when my old neighbour had his DAF with the CVT, you could never tell if he was mowing his lawn or taking a trip out to the shops :lol:

 

Ford did a CVT in the fiesta recently, dunno what happened to that, I guess technology must have moved on a bit.

[davidwort 0]

... When/if they make diesel blocks with lighter alloys and get smart with the fuelling, then they'll rev like petrols......but if they do, you may aswell stick with a petrol as it'll quieter and smoother and the diesel's torque advantage would be lost if they keep reducing the CR.

 

yep, but the diesel engine has had little development in the past 100 years compared to petrol and look at the advancement in just the last 15 years since R&D focussed more on the diesel. Given the very efficient nature of the diesel engine (look how little heat the VW diesel engines generate) I think there's a whole lot more to come from the diesel in the coming years.

[Kevin Bacon 5]

Agreed. Diesels run very cool. In fact, the pug HDI engine needs post burn injection to get the particulate filter up to the required 300 degrees so that it can do it's job!! Kind of ironic that the bain of the petrol engine modder's life is heat, and with diesels it's getting the darn things hot enough!

 

I also love the way TDI diesels will sit there all day long with a rock steady idle (assuming the injectors aren't clogged and rail pressure is up to spec) and won't then feel flat and cr@p when you pull away.

 

Swings and roundabouts with all engines tho....

[bcstudent 0]

The current F1 V8s rev to about 20K (as high, if not higher than a lot of bike engines), which is achieved by ultra low internal friction (tiny piston skirts), ultra light and thin conrods, over square bore/stroke and special cranks, very wild cam profiles (idle speed is 5K), high compression ratio, clever timing etc etc.... you name it, it's all other world exotica. Million quid an engine.....used to be rebuilt every race but new rules state the engine has to last a season now IIRC? F1 cranks cost ~ £150,000 a time and take 8 weeks to make by hand.

According to the current FIA regulations...

 

"There are also restrictions on engine use. Each driver may use no more than one engine for two consecutive Grand Prix meetings. If an engine change is required ahead of qualifying at either meeting, the driver will drop ten places on the grid for that event. If the change is made after qualifying, the driver goes to the back of the grid."

 

Still, two meetings out of one engine in *that* state of tune is a bit special!

[dr_mat 0]

Ford did a CVT in the fiesta recently, dunno what happened to that, I guess technology must have moved on a bit.

 

Too expensive probably..

[CoxyLaad 0]

I dont actually know of a bike that revs to 20 grand kev

[Claret Badger 0]

nice to know those crazy Honda engineers and their

 

They're doing something right

[corradokid 0]

nice to know those crazy Honda engineers and their

 

They're doing something right

 

I believe its actually never had an engine returned due to VTEC failure. Still its an impressive record for honda.

[Kevin Bacon 5]

They've had a number of engine blocks pop, but the actual VTEC unit itself is 100% reliable....or 99.8323% reliable....which is damn good.

 

Unlike BMW's twin VANOS which breaks quite often.

 

Keep it simple, keep it reliable.

[davidwort 0]

They've had a number of engine blocks pop, but the actual VTEC unit itself is 100% reliable....or 99.8323% reliable....which is damn good.

 

Unlike BMW's twin VANOS which breaks quite often.

 

Keep it simple, keep it reliable.

 

Do you know if the Honda engines have variable valve lift?, I thought it was only the BMW's that had that and so no throttle flaps.

 

Only problem I can see with the VTECs is the manual adjustment of valvegear which is apparently quite a hefty cost that a lot of owners were originally unaware of, especially on otherwise cheap cars like the Jazz.

 

David.

[StuartFZR400 0]

Stan - I think you got you answers here already; Dr Mat & Kev both picked up on the high state of tune allowing a higher rev range, which simply put, means less reliability/endurance - which is less of a case on a bike which is much lighter, less resistance&effort. Interesting stuff huh, and I bet you're tempted to get tuning your car now huh; so what if you half the life of your car engine on a low-mileage car. We could talk all day about tuning and what tollerances you can get away with - best left to the experts.

 

just a ps note; with a diesel engine not having spark plugs, they ignite by massive compression force instead and thus explode, right?

[dr_mat 0]

Yep.

[**stan** 0]

I dont actually know of a bike that revs to 20 grand kev

 

 

I had a Honda CBR250RR that red-lined at 19,500 and would bounce off the limiter at 21,000. It was a jap import, looked identical to a fireblade only a lot smaller! Had the most amazing 4 cylinder 4 stroke lump! Camshafts were gear driven to cope with the high revs. This was the best bike I've ever owned (had a few!!). Wasn't the quickest by a long way but sounded absolutely amazing.

[dinkus 10]

http://www.rst-v8.co.uk - an (almost ;) ) F1-spec V8 engine that revs to ~20krpm.

 

It's based on bike engines and will need rebuilding every 20k or so miles...and will set you back the best part of £15k...