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Old 20th August 2006, 21:50   #16
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A Carb Zen is definitely better than the MPFI Zen (for LPG)...not only because it can be tuned accordingly BUT because it has a Compression ratio of 8.8:1 where as MPFI Zen has Compression ratio of 9.4:1.
What has compression ratio got to do with amount of air entering the cylinder? Comp ratio is nothing but the ratio between tdc and bdc.

The cylinder size, air intake and valve timing decide how much air the engine takes in.

It's not possible for a 1.4ltr 10:1 ratio engine to take in more air than a 1.4 ltr 8:1 ratio engine (everything else remaining identical). Whats diff, is the degree to which the air fuel mixture inside can be compressed.

The NHC Idsi has a 1.5 ltr engine with a comp ratio of 10+:1, the OHC Vtec had a 1.5 ltr engine with a comp ratio of 9:1. This doesn't necessarily mean that the 10+:1 engine should be more powerful and thristy on fuel.

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Old 20th August 2006, 22:28   #17
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Originally Posted by Shan2nu
The cylinder size, air intake and valve timing decide how much air the engine takes in.
Firstly…Engine size is same in Carb and MPFI Zen.
Second….Do you mean that if we restrict the air flow the car will still take the air it needs? Or if we install a free flow still the car won’t take the free flow air?
Quote:
It's not possible for a 1.4ltr 10:1 ratio engine to take in more air than a 1.4 ltr 8:1 ratio engine (everything else remaining identical). Whats diff, is the degree to which the air fuel mixture inside can be compressed.
NO…you missing a point here..a 10:1 engine will need a leaner mixture than an 8:1 engine…. What does that mean..?...A higher compression engine will use more air THAN a lower compression engine.


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The NHC Idsi has a 1.5 ltr engine with a comp ratio of 10+:1, the OHC Vtec had a 1.5 ltr engine with a comp ratio of 9:1. This doesn't necessarily mean that the 10+:1 engine should be more powerful and thristy on fuel.
.
There are many more factors to decide fuel efficiency of an engine. Also…its opposite of it. Higher compression ratio===Leaner Fuel Mixture===More FE. Diesel engines have ratio of 20:1 so they are more efficient burners!
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Old 21st August 2006, 00:00   #18
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Second….Do you mean that if we restrict the air flow the car will still take the air it needs? Or if we install a free flow still the car won’t take the free flow air?
Thats why, i've mentioned "air intake" in the list.

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There are many more factors to decide fuel efficiency of an engine. Also…its opposite of it. Higher compression ratio===Leaner Fuel Mixture===More FE. Diesel engines have ratio of 20:1 so they are more efficient burners!
An F1 car runs at something like 12:1 ratio. That doesn't mean it can compete in FE, with a similarly sized engine with a comp ratio of 9:1.

Diesel engines need high compression for a totally diff reason. You cannot ignite diesel with spark plugs or low compression, they require high levels of compression to ignite.

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NO…you missing a point here..a 10:1 engine will need a leaner mixture than an 8:1 engine….
Why so?

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Old 21st August 2006, 01:10   #19
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Just as an added detail, adding LPG to a carb engine is not any more tunable than in a MPFI. The only factor that you can adjust is the ignition timing via setting the bell angle on the distributor (60 degrees ideal, advance for LPG), for advanced or retarded ignition timing. But the ECU does that already, so...

Secondly, LPG has HIGHER octane rating than gasoline. It's the specific density with respect to volume that is lesser than petrol. For higher compression ratios, LPG would actually burn better. The ECU will calculate the burning rate via MAP/oxysensor/knock sensor (check out the mixture ratio via EG temp. and exit velocity), and advance the timing.

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Originally Posted by rohitbagai
So if LPG is fitted in an MPFI engine we can not restrict Air (because when MPFI Zen is on petrol it will run into problems) but we will have to pour in more gas to achieve lower Compression ratio. LPG is delivered as a vapor to the engine.
You cannot decrease the compression ratio by fiddling with the A/F ratio. Compression ratios are altered by hard/machine factors, like port area, runner dia, squish area, piston dome design, etc. Engines with similar bore/stroke designs have different compression ratios due to these design factors, not A/F ratio alone.


For example, in carb engines, when you lean the mixture by turning the pilot screw anti-clockwise and adjusting the main/idle jet, you do not increase the compression ratio. You are just fiddling with the A/F.

@2fast4u: Yes, you're right. There's some logic to the fact that carbs are far more easier to maintain/service, and less sensitive to petrol quality. Therefore, in India, carbs might actually be more sensible, but it cannot overshadow the fact that MPFIs are far more efficient.

Last edited by veyron1 : 21st August 2006 at 01:12.
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Old 21st August 2006, 01:17   #20
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An F1 car runs at something like 12:1 ratio. That doesn't mean it can compete in FE, with a similarly sized engine with a comp ratio of 9:1.
Oh..Why F1?? Please read before posting. I said..FE depend on MANY FACTORS (can be 100s!!!) other than Compression Ratio. The “volumetric efficiency” of every engine is different.

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Diesel engines need high compression for a totally diff reason. You cannot ignite diesel with spark plugs or low compression, they require high levels of compression to ignite.
Its not only that they NEED high compression…but also only they CAN achieve that high compression. There is no petrol engine with 20:1 compression because in petrol engine there is air/petrol mixture that needs to be compressed while in diesel engines its only air that what is compressed. So if an air/fuel mixture is compressed above what its octain level permits..it will burst before proper time and hence will result in severe engine knocking.


Quote:
Why so?
As explained above …as the compression level of petrol cars increases … the same rich mixture will result in knocking because each power stroke will be burning higher compressed mixture. In other words the more compression ratio the more mechanical energy will the engine squeeze from that mixture. So for optimal performance same engine with high compression will require leaner mixture to achieve same power
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Old 21st August 2006, 01:46   #21
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Originally Posted by veyron1
Secondly, LPG has HIGHER octane rating than gasoline. It's the specific density with respect to volume that is lesser than petrol. For higher compression ratios, LPG would actually burn better. The ECU will calculate the burning rate via MAP/oxysensor/knock sensor (check out the mixture ratio via EG temp. and exit velocity), and advance the timing.



You cannot decrease the compression ratio by fiddling with the A/F ratio. Compression ratios are altered by hard/machine factors, like port area, runner dia, squish area, piston dome design, etc. Engines with similar bore/stroke designs have different compression ratios due to these design factors, not A/F ratio alone.
Yes exactly..I actually wanted to say that because of low volume of octaine we need to have even richer gas/air mixture to obtain same level of energy ...but because of new rules...I was not able to edit my post because I am a "newbie" and cant view my post instantly after publishing it
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Old 21st August 2006, 03:11   #22
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As explained above …as the compression level of petrol cars increases … the same rich mixture will result in knocking because each power stroke will be burning higher compressed mixture. In other words the more compression ratio the more mechanical energy will the engine squeeze from that mixture. So for optimal performance same engine with high compression will require leaner mixture to achieve same power
So what are you gaining by doing this? Instead of using a leaner mixture and producing the same amount of power, wouldn't it make more sense to use fuel with the required octane and get more out of the engine?

Quote:
Its not only that they NEED high compression…but also only they CAN achieve that high compression. There is no petrol engine with 20:1 compression because in petrol engine there is air/petrol mixture that needs to be compressed while in diesel engines its only air that what is compressed. So if an air/fuel mixture is compressed above what its octain level permits..it will burst before proper time and hence will result in severe engine knocking.
If you knew that, why mention diesel engines in the first place? There's no point comparing comp ratios of petrol and diesel. We're talking about comp ratios of petrol engines.

Quote:
Oh..Why F1?? Please read before posting. I said..FE depend on MANY FACTORS (can be 100s!!!) other than Compression Ratio. The “volumetric efficiency” of every engine is different.
Exactly, so why say high comp engines are more FE? If at all one engine turns out to be FE, it could be due to all those other 100 reasons and not just the comp ratio.

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Old 21st August 2006, 08:57   #23
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So what are you gaining by doing this? Instead of using a leaner mixture and producing the same amount of power, wouldn't it make more sense to use fuel with the required octane and get more out of the engine?
More efficient cars are what every one wants. Government wants less polution. So the norms are getting more stricter. Higher compression makes possible more leaner mixture. The more the leaner mixture the higher energy can be produced by same capacity engines with different compressions. More compression will produce lesser unburnt fuel. Lesser the unburnt fuel lesser the polution.

In case of LPG there is no pump to take out gas from cylender and pour it in engine unlike the petrol. It depends upon the cylender pressure. Since the cylender pressure reduces as the LPG is taken out of it the engine is fine tuned to proform best with lesser presure of lpg. Lesser pressure will mean more leaner mixture. More leaner lpg/air mixture will result in lesser power (because LPG has lesser volume of octaine) with same comression engine. We only make it richer by reducing air proportion and it will thereby increase gas proportion symply because an engine can only take x volume of mixture every time it sucks. When air flow will be restricted more and more gas will take its place and hence will get sufficient power to run the engine effectively.
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Old 21st August 2006, 11:08   #24
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The Carb vs MPFI debate can take another angle when we bring Variable Valve Timing and lift into picture. As far as my knowledge goes a carb cannot adjust the air fuel mixture for the high lift (longer valve open) scenario when the mixture actualy needs to be richer than the normal engine breathing (thru the standards cams) scenario.

There were some cars available which allowed you to change the air-fuel mixture at the drivers end, but I am sure they were not our everyday humdrum cars. But the VTECS and VVTi cars do this without the drivers knowledge.

Moreover I have read that the Civics latest VTEC system keeps the inlet valve open into the initial movement in the compression stroke. Which reduces the compression losses as they say. But effectively it actually reduces the engine displacement by the stroke length it has already covered in the compression stroke. Hence requiring a different amount of fuel to be injected. Infact the amount of fuel is adjusted real time with the new continously variable systems. Could we do this on the Carb'd cars?

Most manufacturers offering products to the developed markets offer a VTEC (thats Honda, I mean its equivalent system).

With a carb-d car I think the best you can do is adjust the car for lean air-fuel mixture if you want FE (Pass the pollution test) or adjust for a rich mixture for power with the airjet screw and the distributer angle movement. But in modern times, why would we want to do that when the comp will do it for you!
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Old 21st August 2006, 11:30   #25
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More efficient cars are what every one wants. Government wants less polution. So the norms are getting more stricter. Higher compression makes possible more leaner mixture. The more the leaner mixture the higher energy can be produced by same capacity engines with different compressions. More compression will produce lesser unburnt fuel. Lesser the unburnt fuel lesser the polution.
Not always. The BS III Baleno satisfies all the pollution norms but uses a 9:1 compression ratio. A lean mixture in any engine would result in lower power and better emissions. Hence the 3 bhp drop.

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Old 21st August 2006, 12:40   #26
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Originally Posted by 1100D
The Carb vs MPFI debate can take another angle when we bring Variable Valve .............................in modern times, why would we want to do that when the comp will do it for you!
Well explained. Thanks!
But why would ppl still buy petes powerbox for getting more from an engine if the stock ECM was such an efective engine?
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Old 21st August 2006, 13:03   #27
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Quote:
Originally Posted by rohitbagai
But why would ppl still buy petes powerbox for getting more from an engine if the stock ECM was such an efective engine?
Thats because the EM takes up values from look-up tables. They have different parameters for different levels of valve timing/lift. But even then those parameters, for stock cars, are set as conservative, for FE drivability and other such things. Which are freed up by the performance remaps.

Say for example, the City VTEC has the air-fuel mixture for each cam operation defined based on market research about what maximum customers might want out of the car. But that said, the setting can always be changed to run richer for more power and at the same time the settings for ignition timing also would need to be varied, which is where guys like Pete's come in.

You might argue that, in that case how is it different from the screwdriver scenario with a carb'd car, but a carb'd car will not be able to switch between the setting required for varied valve operations (which is becoming more of a norm with every passing day).
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Old 21st August 2006, 13:11   #28
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And doesn't Saab have something called a variable compression engine wher ethe compression ratio is altered depending on engine requirement.

The Jap version of our Accord (Inspire) uses Variable Cylinder Management where a certain number of cylinders can be shut down when there is no need for power and can be brought back when you want to acc.

Carbs used to be good during their time, not now.

A well tuned MPFi will always be more powerful and efficient than a well tuned Carb.

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Old 21st August 2006, 13:22   #29
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Originally Posted by Shan2nu
The Jap version of our Accord (Inspire) uses Variable Cylinder Management where a certain number of cylinders can be shut down when there is no need for power and can be brought back when you want to acc.
Thats a great example. I guess, its the same thing as cylinder de-activation. High capacity engines in US get a tax break if they employ this. (They call it Guzzler tax or something). Its pretty complex knowing that they have to keep all the inlet valves open (correct me here if I am wrong) to allow the de-activated cylinders have their pistons reciprocating freely without putting any compression load on the active cylinders.

Last edited by 1100D : 21st August 2006 at 13:24.
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Old 21st August 2006, 13:46   #30
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Its pretty complex knowing that they have to keep all the inlet valves open (correct me here if I am wrong) to allow the de-activated cylinders have their pistons reciprocating freely without putting any compression load on the active cylinders.
Actually the Honda engine seals the unwanted cylinders complete. There's no valve lift.

The Inspire's 3ltr V6 works like a V6 when you step on the gas and turns into a 3 cylinder 1.5 ltr engine when you're cruising.

Considering it's 240bhp output, shutting down 3 cylinders will still give it around 120bhp from 1.5 ltrs, which is more than enuf for cruising.

And since you're using half the number of cylinders along with sedate valve timing (Vtec), FE is no longer as issue. India being a FE crazy country should have been the first to get this.

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