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-   -   6th Gear for a car ! Need, Criteria, Advantages & Disadvantages (http://www.team-bhp.com/forum/showthread.php?t=88515&page=0)

swiftnfurious 8th September 2010 09:58

6th Gear for a car ! Need, Criteria, Advantages & Disadvantages
 
Guys, searched the forum for this topic, could not find one.

I always felt a need for a 6th gear for my swift -- because of the below reasons.

I drive quite a lot on highways and felt a 6th gear could be added because of the punch Swift D gives in between 2000-3500 rpm. Frankly i have touched a max of 160kmph a couple of times and that was in 3000rpm I think.

I shift to 5th gear around 40-60 kmph [I dont shift gear based on the rpm, but rather the speed] and I sometimes pull it to the reverse gear position thinking that I am in 3rd gear -- because of the power I have in 5th gear as well.

I see that the products that we are getting have started to be implemented with 6 forward gears [i20 CRDi]. And its surprising to see that they never offered a 6th gear for the 1.5L 110 bhp CRDi unit, but for the 1.4L 90bhp unit !:eek:

I know that one advantage of 6th gear is to get better mileage.

Are the maufaturers shying away from this option because of cost? or some other complications ?

I would like to see your views with respect to the below points.

1. Whats the need of a 6th gear in a car??
2. Is there any particular criteria for implementing the 6th gear in a car??
3. Advantages?
4. Disadvantages?

Sheel 8th September 2010 10:20

6th gear acts as an overdrive gear, the gear ratios would be more spread out to accommodate an extra cog.
advantages would be maximizing FE and cruising at low rpm's (relatively)
disadvantages--shifting a wee bit too much?(I do not mind this & is keeping the Car in the power-band rewards you handsomely).

Hyundai did this for (maybe?) novelty associated with 6th gear.

sgiitk 8th September 2010 11:22

For best performance you need to keep the engine rpm between just below maximum torque to just over maximum power. Many engines have a broad power band so may not benefit from a six speed box. On the other hand many engines (esp diesels) have a relatively narrow power band. So here a six speed box helps.

Modern cars may have two or more gears as overdrive gears (i.e. gearing beyond 1:1).

tsk1979 8th September 2010 11:39

Just over max power....?
Many engines make max power near the redline, I do not think an engine should be abused to its redline frequently.
I would say keep the engine from just below max torque band start to the end of max torque band.
For example, an engine making max torque from 1700-2500rpm, I would upshift around 2600rpm, and downshift when the rpm falls below 1600 or so.

sgiitk 8th September 2010 12:02

Quote:

Originally Posted by tsk1979 (Post 2061124)
Just over max power....?
Many engines make max power near the redline, I do not think an engine should be abused to its redline frequently.

The redline is typically about 10% higher than the max power point. Hence what I wrote. To be very precise you should look at the road bhp and then select the band but what I suggested is a good approximation.

If you shift up and the road bhp (at the lower rpm) is lower then it may be beneficial to stick to the lower gear (red line considered of course). If you see the power curves the torque eases off very fast at the top end and so does the bhp!

tsk1979 8th September 2010 12:05

Sorry, I totally disagree.
For example lets take a tata safari 2.2 engine, which redlines around 4500rpm, and develops peak power around 4200rpm.
You are saying upshift only around 3500rpm,
However, for benefit to engine clutch and fuel economy I think its best to go max till around 2800-3000rpm, and ideally keep engine in the middle of the torque band.
For a petrol engine which redlines at 7000rpm, peak power is round 6800rpm.
There also if you let engine climb to 6000rpm before upshifting, its not good.
Its always better to keep engine in max torque band, which is much lower than the point max power is produced.
I am yet to see a torque curve which eases of near the max power ease of point. Max torque usually eases off much lower than the max power(which is often near redline)

sgiitk 8th September 2010 12:17

@tsk1979: Let us agree to disagree.

What I was saying was for best performance stick from below PEAK TORQUE to above PEAK POWER. What you point out - a power band from 3500 (i.e peak torque around 3600 rpm) to 4300 is exactly the reason that Diesels benefit more from 6-speed boxes.

ph03n!x 8th September 2010 12:17

Quote:

Originally Posted by sgiitk (Post 2061092)
For best performance you need to keep the engine rpm between just below maximum torque to just over maximum power.

From a Diesel context, I am not getting this logic. My engine makes its peak torque of 24 Kgm between 1900 ~ 2750 RPM, and power peaks to 110 BHP at 4000 RPM. So where should I maintain the running RPM of the engine - just below 1900 RPM (just below maximum torque) or at 4100 RPM (just over maximum power)? Or is it 4100 RPM, which is above maximum torque, and over maximum power too?

Even in case of Petrol cars, torque output will peak much earlier than power. And am assuming if the Engine BHP is peaking at 4000 RPM, the road BHP too would peak somewhere around that.

SS-Traveller 8th September 2010 12:19

Quote:

Originally Posted by swiftnfurious (Post 2060943)
I always felt a need for a 6th gear for my swift -- because of the below reasons.

...a 6th gear could be added because of the punch Swift D gives in between 2000-3500 rpm. Frankly i have touched a max of 160kmph a couple of times and that was in 3000rpm I think.

I know that one advantage of 6th gear is to get better mileage.

Are the maufaturers shying away from this option because of cost? or some other complications ?

I would like to see your views with respect to the below points.

1. Whats the need of a 6th gear in a car??
2. Is there any particular criteria for implementing the 6th gear in a car??
3. Advantages?
4. Disadvantages?

@snf: What you are proposing is to ask for a 6th gear to maintain higher speed at lower engine rpms, which you think would improve mileage. Assuming your current 5th gear has a drive ratio of 0.8:1, you want one more gear to perhaps give you a drive ratio of 0.7:1. But that is not what a 6-speed gearbox is about.

To answer your questions:

1. The number of gears is determined by the rev band, power and torque curve of an engine. Ideally, each gear at the higher end of the rev range should have an overlap in terms of similarity of power & torque output, with the lower end of the rev range of the next gear. More the number of gears, the more closely spaced the ratios are - thus this overlapping is produced better.

2. Criteria for using 3/4/5/6 (or even more) gear ratios are determined by the characteristics of the engine, as well as the final drive (differential) ratio. In general, 5 gears have been found to be adequate for passenger cars, though 6 are sometimes used. However, the 1st and 5th gear drive ratios in a 5 speed gearbox commonly remain similar to the 1st & 6th gear ratios of a six-speeder. OTOH, an HTV gearbox can well have 8 or 10 speeds because of the low-revving characteristics of its engine, and the high-ratio final drive being used, to help it carry heavy loads.

3. Advantages of a 6-speed GB include more closely spaced ratios, allowing selection of the right gear more easily to let the driver accelerate and drive more comfortably, without the car running out of steam. The 0-100 km/h times would improve with a 6-sp. GB if 2 cars came with everything else equal, except a 5-speed GB or a 6-speed one. However, IF, as you propose, the 6-speed GB had a top gear drive ratio which is lower than the 5th gear of the 5-speeder, theoretically he can achieve a greater top speed.

4. Now for the disadvantages. I said theoretically, because a higher gear puts more load on the engine - with this extra load, the engine may not even be able to rev beyond a certain rpm. Invariably happens - the revs you can do on 4th gear (1:1 is the usual ratio), are not achievable in 5th gear.

Plus, even if the manufacturer did provide a 6th gear ratio which is lower than the 5th of a 5-speed GB, your managing to achieve those higher speeds may not be supported by the car's dynamics. The chassis, suspension, tyres, brakes etc. can take just so much speed, and not more - speeding beyond this increases the risk factors. And of course, the biggest disadvantage is a higher cost.

sgiitk 8th September 2010 12:24

@ph03nix: You go from say 1900-2300 to 4150.

What is power: Torque x rpm x a constant. The constant is irrelevant to the discussion. So the peak power will invariably be at a higher rpm than the peak torque. Quite frankly the peak ttorque at 3500 as indicated by tsk1979 is difficult to believe! I also think it should be more likely in the 2000 rpm area.

Sutripta 8th September 2010 23:24

Quote:

Originally Posted by SS-Traveller (Post 2061200)

3. AdvantagesThe 0-100 km/h times would improve with a 6-sp.
Not if you have a slow gearbox, and if you have to make an extra shift in the 6speeder.

GB if 2 cars came with everything else equal, except a 5-speed GB or a 6-speed one. However, IF, as you propose, the 6-speed GB had a top gear drive ratio which is lower than the 5th gear of the 5-speeder, theoretically he can achieve a greater top speed.

Actually, theoretically, as you have pointed out later, top speed is a function of power available and power required. For a tourer, a designer will normally set up the ratios so that power limited top speed will be available in 5th, and cruise in 6th. Top speed in 6th will be less. For a performance machine, top speed in 6th.

Regards
Sutripta

GTO 9th September 2010 18:49

I absolutely love the 6th gear of my C220. It leads to extremely relaxed cruising on the highway and maximises the tank range (increased kpl) too. Plus, the gear ratios of the C are perfectly matched to the engine's torque characteristics. I'm sure it has something to do with Mercedes having the flexibility of an additional cog.

That said, it's hardly ever used! The only opportunity we have to access it is on the highway. And even then, how many Indian highways can you cruise constantly in one gear? The 6th ratio is great to have, but I don't think it makes any sense for most everyday hatchbacks & sedans.

BTW, there's quite the ego war between luxury car marques on this topic. Mercedes came out with the 7 speed AT, Lexus outdid them with an 8 speed in the LS and now there's news of the next-gen S Class being equipped with a 9 ratio box!!

mxx 18th September 2010 12:13

Quote:

Originally Posted by SS-Traveller (Post 2061200)
because a higher gear puts more load on the engine - with this extra load

Can anyone clarify on this?
Isnt stress on the engine proportional to the rpm?
ie. Is the load on the engine at 3000 rpm at say 2nd or 3rd gear any different from the same engine running at 3000rpm at 5th or 6th gear?

PS: My question is only wrt to engine stress, as I am aware that the other parts in the drivetrain will have more stress due to increased speed.

pranavt 18th September 2010 12:22

Quote:

Originally Posted by mxx (Post 2074151)
Can anyone clarify on this?
Isnt stress on the engine proportional to the rpm?
ie. Is the load on the engine at 3000 rpm at say 2nd or 3rd gear any different from the same engine running at 3000rpm at 5th or 6th gear?

PS: My question is only wrt to engine stress, as I am aware that the other parts in the drivetrain will have more stress due to increased speed.

Higher gear does put more load on the engine. Even ridden a geared cycle? Try 20 rpm with your feet on a low gear and a high gear, you will understand. Takes more power because the cog is bigger and ratio is smaller. 1st gear puts minimum load on the engine and 5th/6th gear puts maximum load.

valhallen.282 18th September 2010 12:58

From a secondary perspective I'm thinking that for a manual transmission cars more gears = more shifting = more clutch usage. While its not bad it can't be good for the mileage or the drivers left foot either. even in quite a few autos its been found that you end up consistently just hunting for which gear to jump to. As it is 5 gears tends to bother plenty of people. Like my dad just CANNOT get the car into 5th. (We had a van before) he says it confuses him how to get to it and out of it!


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