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Efficiency with Additionl traction on demand

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We all know that low-powered engines are more fuel efficient than higher powered ones. Also, a high powered engine is required only while accelerating; Howerever once the desired speed is attained, a low powered engine would suffice. ( A 3 sheeled auto-rikshaw/tuk tuk carries around upton 4-5 people with a mere 125 cc engine.)

Hence I suggest, instead of using a single 2 litre engine, using two engines, one, say, half litre (primary) and other one secondary 1.5 L; Car will have two additional wheels which will be driven only by this 1.5 L engine. These two wheels will hang in the air and will be lowered and make contact with ground only when additional torque is required, such as during accelertion.

When car is starting from 0 to 60 mph, it will be a 2L, 6 wheeled, 3 axle car. Once it reaches constant speed, it will be a half litre, 4 wheeled car. Additional two wheels will be retracted and 2nd 1.5 L engine will stop.

Thus 1.5 L engine can also work as a fail safe engine. If primary goes down in transit, other one can always be used. These additional two wheels can be located some where near rear wheel. Also, if a rear wheel goes flat, additional wheels can be used at a touch of a button.

VJW, Dec 16 2011

"Variable displacement" http://en.wikipedia...riable_displacement
Equivalent to this Idea, just disable some of the cylinders of a high-power engine. This method is also much more efficient than having two separate engines and transaxles. [Vernon, Dec 16 2011]

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       VW used to race Jettas with an engine at each end.
FlyingToaster, Dec 16 2011
  

       Hmmm, 1 V8 for each wheel. Rumble, rumble, rumble...
saedi, Dec 16 2011
  

       The simplest solution would be to connect one engine to the front wheels, and the other to the back wheels, giving AWD without a transfer case, reverting to 2WD when cruising. This was done successfully on Scrapheap Challenge; one engine blew up, and they were able to finish the heat with the remaining engine. It was said that the MOT does not authorise vehicles with 2 engines on public roads.   

       Your additional pair of wheels seems like a waste.
spidermother, Dec 16 2011
  

       There was a 4wd mini moke with 2 engines, each driving 2 wheels.
pocmloc, Dec 16 2011
  

       The driving dynamics resulting from dropping another set of wheels down would be difficult to manage in any sort of smooth transition, and handling would be quite awkward. This vehicle would probably become a complex drive-by-wire system, and even then, the physics may make it ungainly.
RayfordSteele, Dec 16 2011
  

       Without putting too arrogant a point on it, only those who have actually driven a 6x6 vehicle (rear-drive tri-axles do not count) can understand how much an extra set of drive wheels change the dynamics of the chassis (which is why most 6x6s have front-drive cut-outs). At higher speeds, you can get into situations where it seems the truck is trying to drive around itself on inside corners. Having also driven a variable displacement vehicle, I think I can safely say that adding it to the mix would take meticulous care in not only design but also operation.   

       Example for the uninitiated: when turning my duece* off of a forest track onto a paved road with the front drive still cut in, I must ease it into the lane slowly, lest the front wheels, turned to align with the new road, grip the tarmac and start pulling me into the turn whilst the four rear wheels are still pushing hell-bent in a straight line. This will result in the outside front wheel 'hopping' along as I veer into oncoming traffic, followed by a violent lurch when those clawing rear wheels finally grip something solid. If my brake horsepower were to suddenly double or triple at that moment... well, you'd have a giant truck trying to drive in several different directions at once.   

       *I'll grant that a duece is not a representative model for all 6WD vehicles, but it is a fairly common one. It is not a highly sophisticated machine, which is part of its charm, but I think the same problems could arise in other designs, though possibly to a lesser extent.
Alterother, Dec 16 2011
  

       // only those who have actually driven a 6x6 vehicle //   

       The Stalwart, a NATO 6 x 6 amphibian, is ... quite interesting ... to drive at any speed. Considerable skill and anticipation is mandatory, lest thou end up Face Down In A Pool Of One's Own Blood, and on a charge of Wilfully Damaging A Vehicle to boot.   

       // VW used to race Jettas with an engine at each end //   

       Siroccos, too. The front engine had an automatic box, the rear a manual shift. Unbelievably quick, and the extra weight did wonders of the roadholding. Say goodbye to the rear seat (occupoed by the fuel tank) and any pretence at "economy", but still ....   

       Sadly, not road-legal.
8th of 7, Dec 17 2011
  

       //one V-8 for each wheel//   

       Waste of weight. There is only so much torque you can put to the ground to launch with. The rest is just wheelspin, unless you want to burn rubber in 5th gear.
RayfordSteele, Dec 17 2011
  

       [+] because variable wheel properties is a new idea to me. Narrow, hard tires would allow efficient cruising with minimal rolling resistance, while wide, soft tires would allow fast acceleration. This could be accomplished with multiple axles, or with specially designed variable tire pressure tires.   

       The variable engine displacement part is old news, and, I think, a distraction from the true nugget of brilliance in this idea.
sninctown, Dec 17 2011
  

       //specially designed variable tire pressure tires// Hmm, how fast would tyres need to spin and/or how dense would the tread need to be so that the tyre deformed due to centripugal forces strong enough to lift the car? Otherwise a variable width wheel might be the answer. decreased width = increased pressure and/or radius.
marklar, Dec 19 2011
  

       [marklar]: to answer your compound question, I suggest you go to a Top Fuel drag race. Bring calipers and a stopwatch.
Alterother, Dec 19 2011
  
      
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