Author Topic: In a quandary about small cars !  (Read 1535 times)

sparky Paul

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Re: In a quandary about small cars !
« Reply #15 on: February 01, 2019, 09:11:16 AM »
Yes, you can play about with tyre profile, and it does certainly help, but comparing a Jazz and a Corsa is like comparing chalk and cheese. There's no amount of suspension tweaks can hide the fact that the Jazz is basically a Japanese car designed for Japanese quality roads.

I'll admit, it's is a bit of a generalisation, but they just don't have the suspension travel you get on European car designs, which generally cope with our crappy roads better.

andruec

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Re: In a quandary about small cars !
« Reply #16 on: February 01, 2019, 12:36:31 PM »
Then there's that Atkinson cycle business which I don't fully understand but is designed to increase fuel economy. Am I right in thinking that a firm press on the accelerator pushes the car through this? Asking for a friend :P
Hey. The Atkinson cycle uses different valve timings in order to operate more efficiently. Otto cycle is the way most internal combustion engines have always operated. The Jazz isn't a true Atkinson cycle engine but it's a similar principal.

Anyway the Jazz engine seems to operate in Atkinson mode below roughly 2,500 rpm. If you can get the revs above that point you will get better response. In a manual car it's not so noticeable because gear selection and road speed dictate rpm. If you wanted to exit a roundabout quickly in a manual you'd go around it in 3rd or 2nd and then you'll be above 2,500 when you leave. But the CVT breaks that link and - in 'D' mode at least - appears to be engineered to favour low revs.

What I've found is that a quick 'prod' of the accelerator pedal appears to act like a mini-kick down. The ECU will take the hint and bump the revs up. A gradual increase is less effective as it seems that the CVT ignores you at first then suddenly wakes up and takes the hint. It does take a while to get the hang of 'the prod'. It wasn't too hard for me because I've always done it even on older models simply because I don't like to hang about. All the Jazz I've owned have shown a preference for 3,000 rpm when you ask for 'a bit of power' so a quick jab with the Mk3 usually puts you around that mark.

Alternative as some posters here do you can blip the left paddle but then if you're going to use the paddles all the time you might as well not be driving an automatic. Interestingly my last courtesy car was a facelifted Mk3 CVT and it didn't seem to have this problem at all. Possibly Honda tweaked the logic to improve it. I also noticed another change they'd made: If I floor the accelerator my RPMs jump to 4k then gradually rise up to the maximum where they sit for as long as I keep the pedal down. With the face lifted Mk3 they rose to maximum from 4k a lot faster and then started a drop, rise, drop rise sequence like a manual car would. I couldn't feel any gear changes so it must have been clever fakery but it shows that the facelifted Mk3 has had some tweaks to the CVT.

culzean

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Re: In a quandary about small cars !
« Reply #17 on: February 01, 2019, 01:13:02 PM »
Interesting post Andruec. I mentioned above about a resistance to opening the throttle and I think I have this to a degree. A few years driving turbo diesels with that effortless low down grunt may have built up that resistance to opening up the throttle a bit more. It may also explain why some motoring journalists compare the Jazz unfavourably to the small turbo boosted petrol engines in many other small cars.

Then there's that Atkinson cycle business which I don't fully understand but is designed to increase fuel economy. Am I right in thinking that a firm press on the accelerator pushes the car through this? Asking for a friend :P

Originally Atkinson cycle engines used funny variable stroke crankshaft and all sorts of unreliable complicated stuff to vary the compression ratio, the idea being that when the power is not required ( slow acceleration and low revs ) you can make engine more efficient by just using enough compression and fuel so that at bottom of power stroke there is 'nothing left' and all power in charge has been used and non goes down the exhaust pipe.  The modern take on Atkinson ( introduced by Honda on the 8th gen Civic 1.8 vtec petrol in 2006 ) is that you leave the inlet valves open longer and allow some of the charge to be pushed back into inlet manifold, this lowers the compression ratio because there is less to compress.  Due to direct injection on mk3 you are only pushing air back into manifold as the fuel is not injected until the air is compressed and ready to fire ( like a diesel) . Because direct injection don't suffer from pre-ignition ( you are only compressing air, not a fuel / air mixture the compression ratio for full power can be high, more than 13:1

I would hope that depending on throttle position ( how far and fast you press pedal ) the Atkinson would be terminated early or never requested, whereas at low throttle openings it can help to improve mpg.   The other thing Atkinson helps with are pumping losses due to pistons sucking against an almost fully closed throttle butterfly at low throttle opening, because power can be controlled by compression ratio and amount of fuel injected and throttle butterfly is servo controlled and not physically connected to the pedal anymore the butterfly can be opened wide and engine will not immediately go to maximum revs, but because power is limited by CR and fuel, the load on the engine will limit revs.
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Jocko

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Re: In a quandary about small cars !
« Reply #18 on: February 01, 2019, 01:36:32 PM »
Does anyone other than Honda currently use the "Atkinson cycle"?

mikebore

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Re: In a quandary about small cars !
« Reply #19 on: February 01, 2019, 01:57:38 PM »

Alternative as some posters here do you can blip the left paddle but then if you're going to use the paddles all the time you might as well not be driving an automatic.

Good post apart from this statement! IMHO there is a massive difference between driving an automatic with an occasion flip on the left paddle, and driving a full manual. The occasional flip might represent 0.1% of driving time but a full manual is 100%.

DaveBerks

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Re: In a quandary about small cars !
« Reply #20 on: February 01, 2019, 02:16:43 PM »
Interesting comments all, many thanks from a non-techi person like me :'(

The ingenuity of the engine in the Mk3 (mine is pre-facelift) i.e. Atkinson and Otto (strange bedfellows !) is remarkable but asking the car to move smoothly and intuitively between the two makes me think this is not a marriage yet made in heaven !

Atkinson doesn't want you to leave, you have to 'put your foot down' sharpish and ignore its protesting screams to visit the more exciting Otto and then relent and return to Atkinson who sips petrol like its the most precious nectar there is. The result is something close to a Greek Tragedy for the poor adulterous driver. That little glowing green to blue indicator on the dash remains you of your misdemeanours in drinking too much fuel with Otto (like a normal driver). 'Get back' it flashes or it really is a divorce this time ! "I just wanted some fun" you say "please forgive me. Promise I won't do it again".

I'm joking and exaggerating a little bit but I do think the poor hapless non-technical driver needs better explanations and support from the dealers and Honda on this.  I applaud the engineers' daring and creativity but someone at Honda needs to keep their feet on the ground and their sales people should explain more what Honda are doing and why it is such good idea, as this might avoid some serious misunderstandings later on as well as prevent the start an anti Honda club ! Equally it's clear that motoring journalists can't be relied upon to understand the cars they are reviewing. Most it seems are simply fashion journalists.

I better understand the car now, thanks, I appreciate the perfect quality of Japanese road surfaces, the unbelievable ingenuity of Japanese engineers at Honda and their attempts to design cars which are 'close to the edge' and very clever.....yes...very clever !  ;D.   Just think about the poor customer a bit more please Honda  :-[



andruec

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Re: In a quandary about small cars !
« Reply #21 on: February 01, 2019, 03:26:48 PM »
Does anyone other than Honda currently use the "Atkinson cycle"?
Yes, several cars according to Wikipedia.


Jocko

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Re: In a quandary about small cars !
« Reply #22 on: February 01, 2019, 04:01:09 PM »
Very few non-hybrid models. Obviously the hybrid motor gets round the problems reported here, with the non-hybrid Jazz. Mind you, when I had the 67 plate, manual SE on loan, I wasn't aware it had "Atkinson cycle", and it felt pretty much the same as my own car.

culzean

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Re: In a quandary about small cars !
« Reply #23 on: February 01, 2019, 04:16:17 PM »
http://innovatize.blogspot.com/2012/01/interesting-technology-by-honda-i-vtec.html

Here is an interesting article on later direct injection honda engines ( i-VTEC-i ) - it seems that at light loads / low revs the fuel is injected directly into piston bowl during the compression stroke - this enables really lean fuel ' air ratios of 65:1 ( with a corresponding decrease in power)  but during large throttle openings / high revs the fuel is injected during intake stroke where they go back to 14.5:1 ratio.  The engine in the video ( bad quality ) about 3rd one down is a 2 litre fitted to the Honda stream minivan, and article is dated 2012 - don't know if this same system used on newer Jazz engines where they make the air/ fuel extremely lean at low loads and revs and that will explain the lower power rather than the atkinson cycle which is not mentioned on the i-vtec-i DI engine video.  They do mention the 12/16 valve system where one of the intake valves can be left closed at lower revs.

A quick overview of Jazz direct injection engine from New Zealand,  this does mention both direct injection and atkinson cycle in the same engine.

https://www.honda.co.nz/technology/engine/jazzearthdreams/

This is an article on the original atkinson / otto engine in 8th gen Civic 1.8 i-vtec

http://asia.vtec.net/Engines/RiVTEC/

Mazda skyactive engine also worth a look

https://www.popularmechanics.com/cars/car-technology/a20650087/skyactiv-x-engine-mazda/
« Last Edit: February 01, 2019, 04:30:04 PM by culzean »
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Downsizer

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Re: In a quandary about small cars !
« Reply #24 on: February 01, 2019, 07:55:58 PM »
Thanks all for your comments.

It's true that the Corsa has 15 inch wheels whereas Jazz EX has 16 inch, but why are the 16 inch wheels thinner (less  contact with the road surface) than the 15 inch ?...is this to save the manufacturer money ? The 16 inch look better since 15s can look lost in the wheel arch. But this gives manufacturers no excuse for poor ride comfort in my opinion. 
I don't think the 16" tyres are "thinner" (I.e. narrower tread). They have less depth because there is one inch less rubber diameter to compensate for one inch more metal.  That's why the ride is different.

Downsizer

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Re: In a quandary about small cars !
« Reply #25 on: February 02, 2019, 04:16:27 PM »
I currently driving a Vauxhall Corsa 1.4 automatic while my Jazz EX 1.3 auto is in for repair...for some reason other drivers like to drive into it when it is stationery !  Tailgating seems to be an everyday occurrence in the South East where everyone is in a hurry and safe gaps between cars are a thing of the past.

What I notice is how comfortable the suspension is in the Vauxhall, riding over bumps and seams, and how planted the car feels on the road and how responsive the engine is compared to the Jazz which makes me feel I'm thrashing it to get anywhere over 20 MPH and crashes over bumps and seams on the normally poor road surfaces we now have to pt up with. I suspect the car is very light in order to save fuel.
I'm currently driving a hired Corsa in Lanzarote and I notice it is fitted with 195/55 x 15" tyres compared with 185/60 x 15" on my Mk 3 SE Jazz.  I don't know if this is the same in the UK - it may to cope with some of the un-metalled minor roads here.  Personally, I don't much like the car.  It feels heavier and the ride is a bit "wallowy"- does anyone remember the old Zodiacs and  Veloxes?  Also, even with driver's seat fully raised" I feel a bit low, although I am 5' 9". But these are very subjective issues.

sparky Paul

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Re: In a quandary about small cars !
« Reply #26 on: February 02, 2019, 09:37:59 PM »
I'm currently driving a hired Corsa in Lanzarote and I notice it is fitted with 195/55 x 15" tyres compared with 185/60 x 15" on my Mk 3 SE Jazz.

The 195/55x15 tyres on the Corsa would theoretically be slightly stiffer, but there's very little in it. I think it was Jocko who mentioned the differences in tyre brand earlier, and in this case, a more pliable tyre could sway it either way.

RichardA

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Re: In a quandary about small cars !
« Reply #27 on: March 12, 2019, 01:56:57 PM »
I think you will find the Corsa is a much newer design than the Mk 3 Jazz. How do the mileages compare between the two vehicles? Both can have an effect on ride comfort.
Hard to say, the current Corsa came out in 2016 but is heavily based on the previous gen from 2006.

Japanese cars are normally less good at NVH than European cars.

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culzean

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Re: In a quandary about small cars !
« Reply #28 on: March 12, 2019, 02:31:19 PM »
Japanese cars are normally less good at NVH than European cars.

I think Japanese are always aiming for lowest possible vehicle weight and may skimp on the sound deadening material.  Anyway if it is a choice between slightly higher road noise and much better reliability I guess I will go for reliability every time and buy the quietest tyres I can afford- I think Vauxhall are better now than they used to be, but I had a couple of Vauxhalls years ago and was decidedly not impressed, my first Ford ( Sierra ) and later Japanese cars were many orders of magnitude better.
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AlanTR

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Re: In a quandary about small cars !
« Reply #29 on: March 14, 2019, 07:44:11 PM »
I recently hired an Opel (Vauxhall) Mokka Turbo (petrol) recently in Fuertaventura. Nice enough car but, in my opinion based on 3 days driving, nowhere near as good as the Jazz.

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