Aug 11, 2007

Bajaj DTS-Si: What it cannot do

Could Bajaj make a super-frugal scooter with DTS-Si?
Given that DTS-Si is a tech-idea that works primarily on the engine head, there's no reason why this would not be possible. However, a 'wasteful' variomatic transmission would more or less put paid to all the efficiency benefits gained from the technology and render the application pretty much without major benefits.

Could there be a DTS-Fi-Si?
Well, no. You see, a powerful, high-revving engine is usually optimised to produce big power and big revs. At high revs, you need each cycle to get over quickly so the next one can begin. Which means the engine head is designed for flow lots of mixture (greater volumetric efficiency), rather than produce a super-clean burn on a lean mixture at low flow speeds (greater combustion efficiency). That's why the next DTS-Fi evolution has more chance of being a four-valve head (two intake valves flow more air-fuel mix than one) than a DTS-Fi-Si.

Related links:

5 comments:

Glifford said...

'wasteful' variomatic transmission

My 110 cc (8 BHP?) Wave gives a lower kmpl figure than my 180 cc (16BHP) Pulsar! :P

But benefit is benefit, even if it is miniscule!

Only wish Bajaj had got those auto transmission (3-speed) on the Saffire right the first time. It was more efficient than a vario anytime.

Finally they gave up and made the vario Wave!

JulianPaul said...

i thought CVTs were very efficient. something about being in the optimum ratio for any given speed/load condition. or are those CVTs different from the varios used in our scoots ?

Rahul said...

I've been reading your blog for some time now.... right mix of new bike details and technical articles to keep you plastered on to the screen... great going....

The DTS-Fi-Si is a beautiful concept/topic I thought it needed further discussion. This has been made possible before by both Honda(the famous VTEC) and BMW(double VANOS techn).

The prerequisit for fusion of these two technologies would fisrtly require a 3/4 valve cylinderhead.

Basic Idea being inducing swirl and hence turbulance at lower RPM (by closing one valve - during intake - in a 3/4 valve per cylinder engine OR by lifting the intake valve/s late during intake.)
This would ensure a fuel efficient engine at low rpm.
At higher RPMs, air flow is increased (by lifting the 3rd/4th valve or advancing the intake valve opening, or increasing the valve lift.)This would ensure a very powerfull motor at higher RPM as well.

An example for such a motor in the motorcycling world would be the HONDA VFR 800

But as you had mentioned, the logical upgrade to DTS-Fi would be a normal 3/4 valve engine. Bajaj would need more time and R&D to produce such technology especially for a single cylinder engine.

But if produced, It would be the equivalent of a pulsar 150
that gives 50+ km/ltr in normal riding conditions and producing around 17-18 bhp at 13000+ rpm (ofcourse with a noticable drop in fuel efficiency).

More juicy thoughts like a 400 cc 4cylinder engined bike with DTS-Fi-Si or whatever enter my mind at this point... :)

Arun Murali said...

Hi Rahul,
Some thing like DTS-Fi-Si is already going to be out. The CCVTi from TVS is doing precisely what you are talking about except for the FI part. This the bike may gain over time. BTW VTEC is much more costly and may never get into 150cc bikes for the reason that they may cost half the bikes cost.

About Variomatics, they are much more efficient but the problem in Indain variomatics is that they are driven by weights(centrifugal).
They can be tuned to behave only one way. This they choose by selecting something between performance and efficiency. If of all we get a electromagnetic control for CVTs with a computer control or fuzzy logic(may cost about 5-6k extra) it can give you much more than their geared counter parts along with better acceleration.

Rahul Nair said...

"What happens is that TVS' carburettor opens into two vertically stacked manifolds that lead to the two intake valves, one each. As the carburettor slide rises on throttle application, the fuel-air mix first begins to travel down the lower one, the 'swirl port' as TVS calls it. This port is designed to operate with maximum swirl, generating maximum possible turbulence to allow the engine to offer great economy and torque. As the revs (and the slide) continue to rise, the carburettor begins to feed the motor through the other manifold, the 'power port'. This port is straigher compared to the curvy swirl port, and it's job in life is to flow more mixture. At high revs, this allows the engine to breathe deeper and make more power."

@Arun Murali...
:)
well its true I never guessed an indian manufacturer would come up with something like this so fast...CCVTi from TVS i guess works on the same principle... Its got 2 intakes, 1 exhaust and 2 plugs.
Which means the engineers have enough space in the cylinder-head to play around with 3 valves, 1 plug and a direct fuel inector (if they are in the mood).

... Apache in the current set up is 160cc and has an oversquare engine... that is more room in cylinder head than the flame....

@rearset...
Just confirming that i read right... does the second valve remain closed at lower revs and open up at higher rpm for deeper breathing?
It sounds good for things to come if this is the case...

pls post more material on the working of the engine if you get some details from TVS.