RE: Infiniti's variable compression ratio engine
Discussion
zeppelin101 said:
Turbobanana said:
The diagram seems a little confusing too: surely the "power" setting is the one with the higher compression ratio and the "efficiency" the one with the lower?
No because the higher compression ratio yields a corresponding increase in thermal efficiency as defined by the formula:Efficiency = 1 - 1 / (CR ^ (gamma-1))
Where gamma is the specific heat capacity of the fluid in question (straight air is 1.4, air/fuel mix is typically around 1.3).
As a result, the efficiency of an 8:1 engine assuming gamma is 1.3 is ~ 46.5% however at 14:1 it's 54.7%. So an improvement of 9.2%.
That doesn't really give a fair overview of what happens everywhere in the operating map though. For instance, it might be possible to realise all of that benefit at low - medium loads (the region depends a lot on cam profile and supporting technology) however if the higher compression ratio starts to generate knock so that peak combustion pressure is not taking place at the most efficient point of the cycle then the CR should be lower for that point. With VCR - you can do exactly that.
This is the problem that a lot of high CR, high output fixed geometry engines are having now. If CR is high, then along the full load line there is likely to be a significant impact on the knock limit that won't be seen on a much lower compression ratio engine. This leads to an increase in overfuelling (with the associated reductions in efficiency), high demand on the cooling system, high demand on the boosting system etc etc. This is where the likes of "miller" cycle engines come into their own, but they have significant impacts on turbocharger selection. An engine operating "miller" cycle uses a shortened intake cam duration (i.e. it closes some time during the intake stroke, not after BDC as per a traditionally cam'd engine). The benefit of this is that it enables a higher geometric compression ratio to be used which helps increase expansion efficiency (i.e. the amount of work extracted from the expansion of combustion gas is higher than at a lower compression ratio). A side effect is that it also reduces the effective compression ratio (i.e. the amount of compression work done on the gas is reduced because the pressure in the cylinder reduces from intake valve closing to BDC and does not recover until the same point in terms of crank angles (ish) on the compression stroke - a picture paints a thousand words...) which can help with the knock limit somewhat.
How big the benefits are with VCR will depend quite a lot on supporting technology in terms of pumping reduction (i.e. continuously variable valve timing/lift or external EGR). Higher compression ratio typically allows you to get away with dethrottling the engine more to improve the benefit of those technologies.
As long as the combustion efficiency isn't impacted.
A lot of consultancies are developing these systems which are either multi-stage switching or continuously variable.
There is alot I've missed out tbh, I could write a book on all of this stuff...!
Vitorio said:
Turbobanana said:
The diagram seems a little confusing too: surely the "power" setting is the one with the higher compression ratio and the "efficiency" the one with the lower?
.
No, high compression ratio gets the most energy out of the fuel, hence is very efficient, but you cant make a lot of bang, as loads of fuel/boost in a high compression cylinder will pre-detonate, the low CR setting will allow for humongous boost, and thus more power. Go check some specs, compare the CR on a mazda skyactiv engine Vs a high power turbo lump..
Im not entirely sure if the stroke stays the same length, ill need a few more minutes to figure that one out (took me a sweet while to work out which bits moved what way to begin with)
EDIT: i think the stroke is fixed, but i'd have to try the concept at home with some lego technic to really get to grips with it.
Edited by Vitorio on Monday 15th August 12:09
With direct injection the fuel is not in the cylinder until needed, hence no pinking. Also this has a charge cooling effect AFAIU.
You effectively achieve variable compression with a Turbocharger for most of the cycle anyway.
I may have missed something obvious, but that was my understanding.
annodomini2 said:
I thought this was the need from the '80s with Carburettor or indirect injection setups.
With direct injection the fuel is not in the cylinder until needed, hence no pinking. Also this has a charge cooling effect AFAIU.
You effectively achieve variable compression with a Turbocharger for most of the cycle anyway.
I may have missed something obvious, but that was my understanding.
Injection still takes time, it not an instantanious "poof, there is now fuel in your cylinder" effect.With direct injection the fuel is not in the cylinder until needed, hence no pinking. Also this has a charge cooling effect AFAIU.
You effectively achieve variable compression with a Turbocharger for most of the cycle anyway.
I may have missed something obvious, but that was my understanding.
And turbo/super charging doesnt effect the CR, only the total pressure at the top of the stroke, which is why often turbo engines have a lower CR to bring that final pre-spark pressure back within non-pinging limits
otolith said:
Yeah. Like that turbocharging and variable valve timing and fuel injection and overhead valves and electronic ignition and... All made up by marketing, not designed by engineers to fix the basic shortcomings of a simple carburetted sidevalve engine...
Umm... not sure if you are being sarcastic or not.We are already at a great point technology, I do not see this being of much use in the real world.
Especially in a size/space/parasitic drag vs economy point. Plenty of cars can do remarkable economy figures without this nonsense.
A turbocharger for economy is not particularly heavy nor does it provide much in the way of extra moving components... compared to this.
Off to tootle to the shops in my 2 valve Austin 11.
I'd hate to be the one having to map one of these, just think of the variables!
It's astonishing to think this has not been done before, the mass production engine is so so far far behind engineering possibilities. Variable compression ratios were talked about in the early era of automobiles yet nothing has come to fruition. Even my 1940's Ricardo research engine book it's mentioned how much better an could be would be!
It's astonishing to think this has not been done before, the mass production engine is so so far far behind engineering possibilities. Variable compression ratios were talked about in the early era of automobiles yet nothing has come to fruition. Even my 1940's Ricardo research engine book it's mentioned how much better an could be would be!
Anyone else think its a few years too late ?
The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
renrut said:
Going off that picture it suggests to be about twice the size of an existing inline engine. I can't see that being sensible unless it can give some quite staggering benefits. Maybe it'll be another "20V" thing where it doesn't really do much in 99% of cases but its a great marketing tool.
Agreed seems like it would only be suitable in SUV's or RWD applications not something that can be used in most FWD hatchback / saloons."an actuator arm (controlled by the 'Harmonic Drive') rotates a control shaft in the combustion chamber."
I don't think the control shaft is in the combustion chamber. The variability is all in the bottom end of the engine. Basically it looks like there is another link between the big end bearing on the crankshaft and the bottom of the rod. This link is rotated by the actuator, changing the angle which then changes the effective length of the rod. Longer rod = forces the piston higher into the cylinder (closer to the combustion chamber) = higher compression ratio.
--- vvv this part was wrong vvv ---
The cunning thing is that the actuator itself has to rotate in harmony with the crankshaft to keep the rod at a fixed angle as the engine turns, then it changes this slightly when it needs to alter the angle, and hence alter the compression ratio.
I don't think the control shaft is in the combustion chamber. The variability is all in the bottom end of the engine. Basically it looks like there is another link between the big end bearing on the crankshaft and the bottom of the rod. This link is rotated by the actuator, changing the angle which then changes the effective length of the rod. Longer rod = forces the piston higher into the cylinder (closer to the combustion chamber) = higher compression ratio.
--- vvv this part was wrong vvv ---
The cunning thing is that the actuator itself has to rotate in harmony with the crankshaft to keep the rod at a fixed angle as the engine turns, then it changes this slightly when it needs to alter the angle, and hence alter the compression ratio.
Edited by samoht on Monday 15th August 21:40
J4CKO said:
Anyone else think its a few years too late ?
The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
If they can make it work reliably, then it might change the game materially. If you had an engine that could do 70+ MPG on the motorway cruise at some bonkers compression ratio, and still provide real grunt when called upon, then it would be a pretty attractive proposition to long distance drivers who don't really fancy the idea of enforced tea breaks every few hours, and anyone who lives in a flat who can't charge an electric car.The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
The engineering complexity isn't that hard - the big end is identical, apart from the fact that it pivots (tried and tested shell bearings will be fine) and the actuation mechanism is not stressed that much - piston rod bearings take much more of a hammering and rarely fail.
If you look at efficiency, a really good car engine today is about 20%. Best case power station approaches 60%. Thus, even with all of the losses, burning gas to make electricity is more efficient. There is nothing in the laws of physics to say that a car engine cannot be more efficient than it is today.
Lob this in a hybrid drive train and you could get a really clever solution. The engine can optimise for power generation (40%+ efficiency? No idea what this can achieve), and then optimise for power when you put your foot down. Regen braking captures the deceleration. You might end up with a car that is more efficient than an EV powered by a conventional power station.
xjay1337 said:
Umm... not sure if you are being sarcastic or not.
We are already at a great point technology, I do not see this being of much use in the real world.
Especially in a size/space/parasitic drag vs economy point. Plenty of cars can do remarkable economy figures without this nonsense.
A turbocharger for economy is not particularly heavy nor does it provide much in the way of extra moving components... compared to this.
Off to tootle to the shops in my 2 valve Austin 11.
May have contained sarcasm We are already at a great point technology, I do not see this being of much use in the real world.
Especially in a size/space/parasitic drag vs economy point. Plenty of cars can do remarkable economy figures without this nonsense.
A turbocharger for economy is not particularly heavy nor does it provide much in the way of extra moving components... compared to this.
Off to tootle to the shops in my 2 valve Austin 11.
We could have said "job done, no need for further development" at any point, but clearly there were still advances to be made. See the post further up explaining exactly why these systems to vary CR have been a subject of engineering interest for decades. To say it's a marketing gimmick does a real disservice to the engineers behind this.
cptsideways said:
I'd hate to be the one having to map one of these, just think of the variables!
It's astonishing to think this has not been done before, the mass production engine is so so far far behind engineering possibilities. Variable compression ratios were talked about in the early era of automobiles yet nothing has come to fruition. Even my 1940's Ricardo research engine book it's mentioned how much better an could be would be!
Hardly, ICEs have made humongous strides since the 40s, both in efficiency, specific power, durability etc.. I think in many cases there were easier ways to improve performance (on various scales) then VCR.It's astonishing to think this has not been done before, the mass production engine is so so far far behind engineering possibilities. Variable compression ratios were talked about in the early era of automobiles yet nothing has come to fruition. Even my 1940's Ricardo research engine book it's mentioned how much better an could be would be!
Looking at this schematic, i think this is just getting waaaay out there in terms of mechanical complexity and potential for failure.. If you want high peak power but also efficient chemical energy > motion conversion, i think electric drive with a highly optimized ICE generator, and a battery/capacitor to absorb spikes is the obvious solution these days.
J4CKO said:
Anyone else think its a few years too late ?
The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
As soon as we can charge a car up in under 10 minutes you can nail the final nail in the combustion engines coffin.The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
Mass manufactured cars will almost certainly drop them in favour of a so called eco friendly electric engine. Which in reality just means customers would buy them to save money at the pumps and initially on road licence.
The interesting thing is, if this did happen, and say in 10 years time, 60-70% of cars on the road are EV's, would the tax relief continue? I doubt it.
Surely the most simplistic way of making petrol engines more efficient is better use of the cylinders. IE, if you're pootling along why not drop down to 2, or even just the 1 cylinder.
I have a 3.0D BMW. Although it claims it'll do 47mpg on the combined run it's more like 38mpg at best. It could easily do better if it shut down 3 cylinders when you drop it into eco mode rather than whatever it actually does which hardly saves fuel but makes it painfully slow.
I have a 3.0D BMW. Although it claims it'll do 47mpg on the combined run it's more like 38mpg at best. It could easily do better if it shut down 3 cylinders when you drop it into eco mode rather than whatever it actually does which hardly saves fuel but makes it painfully slow.
PHMatt said:
The interesting thing is, if this did happen, and say in 10 years time, 60-70% of cars on the road are EV's, would the tax relief continue? I doubt it.
Off course not, tax reliefs for hybrids/EVs in the netherlands are already being wound back, the eco diesel is no longer able to get any kind of financial bonus.If anything, road tax or initial tax when buying the vehicle will go up, since income from petrol tax will go down.
J4CKO said:
Anyone else think its a few years too late ?
The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
This.The problem for IC engines is they are too complex and inefficient anyway, this adds even more moving parts.
I think the rise of the EV will put paid to much new development in IC engines fairly soon, this feels a bit like improvements to VHS video recorders.
Renault/Nissan/Infiniti would have done better investing the R&D funds in better motor or battery tech.
PHMatt said:
Surely the most simplistic way of making petrol engines more efficient is better use of the cylinders. IE, if you're pootling along why not drop down to 2, or even just the 1 cylinder.
I have a 3.0D BMW. Although it claims it'll do 47mpg on the combined run it's more like 38mpg at best. It could easily do better if it shut down 3 cylinders when you drop it into eco mode rather than whatever it actually does which hardly saves fuel but makes it painfully slow.
Cylinder deactivation is actually quite complex, its not a case of just cutting the spark and fuel you have to be able to remove any compression from the deactivated cylinder or stop them turning altogether so either a complex valve operation for the entire engine or a 2 piece crank with a clutch type set up. I have a 3.0D BMW. Although it claims it'll do 47mpg on the combined run it's more like 38mpg at best. It could easily do better if it shut down 3 cylinders when you drop it into eco mode rather than whatever it actually does which hardly saves fuel but makes it painfully slow.
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