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Old 11th November 2009, 20:39   #106
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@Shan2nu
glad you pointed out about the distance part( I had'nt thought about it). so is the distance from the axis to the point where force is applied always kept at maximum in all vehicles so that the torque developed due to the distance part atleast is always maximum ?
It depends on what that engine is being used for. An engineer can design an engine based on the manufacturers requirements.

A 2ltr performance engine will concentrate more on having a big bore and short stroke, while a 2ltr engine meant for lugging people around will concentrate on a higher stroke and smaller bore.

A big bore x short stroke engine can revv higher since the piston has to cover a lesser distance between BDC and TDC, but on the downside this engine will achieve its torque and power at higher rpms. But since bhp is calculated with torque & rpm, the higher rpm figure helps the engine generate more bhp.

A small bore x long stroke engine wont be able to revv much but it will be able to produce a stronger torque curve at low rpms.

Increasing number of cylinders for a given engine capacity also helps revv the engine higher since you need smaller/lighter pistons for each cylinder.

The 2.4 ltr 4 pot engine in the Accord has a 87mm bore and 99mm stroke while the 2.4ltr V8 in the F1 car can achieve the same engine capacity with larger 98mm bore and a much smaller 39.7mm stroke. And as i've mentioned before big bore x short stroke = higher rpm/power output.

But using FI has bcom a trend these days since it allows a low revving engine to produce insane amounts of torque and achieve similar power output as compared to high revving NA machines. Like its been mentioned in the above post that the FI EVO X makes 145bhp/ltr@6800rpm, had that been an NA engine, it would have to produce max power beyond 10,000 rpm to achieve 145bhp/ltr.

This might help you understand better - Stroke ratio - Wikipedia, the free encyclopedia

Shan2nu

Last edited by Shan2nu : 11th November 2009 at 20:51.
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Old 11th November 2009, 22:56   #107
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Originally Posted by Shan2nu View Post
I didn't say BHP is relative to engine rpm.

I said BHP its relative to engine rpm only in "NA" form.

Shan2nu
The torque / bhp equation remains the same regardless of the air intake method.

Whether the engine is natually aspirated or turbo-charged or super-charged engine, the equation still remains the same.

Also you should remember BHP is just a derived measure.

As far the compression ratio, you explained it in a different manner, what i was trying to say. For diesel to burn it needs to be compressed to a higher pressure. In order to achieve that you need to design a long stroke engine, which inturn limits the rpm you can reach.

One can still design a high rpm diesel engine but at that rpm and with such high compression ratio, the engine head & block has to be made so strong to withstand the impact of each combustion cycle. This will result in a super heavy engine, which will outweight power gain due to high rpm.

Last edited by oss : 11th November 2009 at 22:59.
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Old 12th November 2009, 08:00   #108
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Originally Posted by oss View Post
The torque / bhp equation remains the same regardless of the air intake method.

Whether the engine is natually aspirated or turbo-charged or super-charged engine, the equation still remains the same.

Also you should remember BHP is just a derived measure.

As far the compression ratio, you explained it in a different manner, what i was trying to say. For diesel to burn it needs to be compressed to a higher pressure. In order to achieve that you need to design a long stroke engine, which inturn limits the rpm you can reach.

One can still design a high rpm diesel engine but at that rpm and with such high compression ratio, the engine head & block has to be made so strong to withstand the impact of each combustion cycle. This will result in a super heavy engine, which will outweight power gain due to high rpm.
I agree with OSS here. BHP being a derived measure is not a true representative of the power of the engine which is the torque. Even though the torque/BHP is calculated at the flywheel, it is not the true representative as there are mechanical losses in power transmission to the wheel.

Anyways I think BHP is utilised heavily to give a layman the basis to compare two engines.

Also one cannot compare the BHP figures of petrol and diesel engine as each one makes max torque is totally different range of RPM for otherwise similarly speced engine.
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Old 12th November 2009, 08:04   #109
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What is really important is to find out and keep in mind the RPM at which the max. torque and max. bhp are produced. If one keeps the engine between those two points in the tacho, the car will perform better. At the very least one should shift up at the max. bhp point.

Cheers,
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Old 12th November 2009, 09:50   #110
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Power = Torque X Angular Speed = Torque X Engine RPM (ignoring units for simplicity)

This equation does not change whether the car is NA or FI. What changes is the torque at a given RPM. This torque is different in NA and FI because the fuel burns at different rate and does not release same amount of energy.

When a car burns 1 litre of fuel, the energy available to do work is same whether it is long bore, short bore, NA or FI. But what happens in FI is that more fuel is burnt per cycle since more oxygen is available for combustion. This increases the available torque per cycle. This is how an FI car can increase power without increasing engine RPM.

Now if you increase the bore size and shorten the stroke, the torque decreases slightly because of the short stroke but the angular speed increases. In other words fuel is burnt at a faster rate. The net effect is that power output increases. This also means that peak power and torque now occurs at a slightly higher engine RPM.

Theoretically it is possible to design an engine to have its peak torque at any given engine RPM. But at higher RPMs there is more frictional losses so there is a compromise.
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Old 12th November 2009, 13:12   #111
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Quote:
The torque / bhp equation remains the same regardless of the air intake method.

Whether the engine is natually aspirated or turbo-charged or super-charged engine, the equation still remains the same.

Also you should remember BHP is just a derived measure.

As far the compression ratio, you explained it in a different manner, what i was trying to say. For diesel to burn it needs to be compressed to a higher pressure. In order to achieve that you need to design a long stroke engine, which inturn limits the rpm you can reach.

One can still design a high rpm diesel engine but at that rpm and with such high compression ratio, the engine head & block has to be made so strong to withstand the impact of each combustion cycle. This will result in a super heavy engine, which will outweight power gain due to high rpm.
Quote:
I agree with OSS here. BHP being a derived measure is not a true representative of the power of the engine which is the torque. Even though the torque/BHP is calculated at the flywheel, it is not the true representative as there are mechanical losses in power transmission to the wheel.
Guys, where have i mentioned torque in the equation? I was talking about bhp and rpm. An FI engine can produce lot more bhp at low revvs as compared to a NA engine.

You can't make a 2ltr NA engine produce 200bhp@6000rpm but you can make a 2ltr FI engine do it.

A Veyron does 123bhp/ltr@6000rpm, try and find a NA engine which can do this, you wont.

Quote:
This equation does not change whether the car is NA or FI. What changes is the torque at a given RPM. This torque is different in NA and FI because the fuel burns at different rate and does not release same amount of energy.
Exactly, which is why FI engines are able to produce high amounts of torque at low rpms which equates to a high bhp figure since bhp = torque x rpm / 5252.

Whether you have 200lbft@2000rpm or 100lbft@4000rpm the resulting bhp in both cases is 76bhp.

This explains why Jitu in his garage is able to produce 250bhp/ltr@9200rpm while the F1 engineers need 18000rpm to produce just 50bhp/ltr more!!!

F1 cars dont revv that high by choice, they have to do it in order to produce that much specific output. But FI machines are not bound by this rule, if they want a certain bhp/ltr at a certain rpm, such an engine can be designed.

145bhp/ltr@6800 is unheard of in the NA world but FI engines do it.

Shan2nu

Last edited by Shan2nu : 12th November 2009 at 13:23.
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Old 12th November 2009, 16:20   #112
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From a novice, i understand that the rpm figure for bhp and torque would be different, how would one keep the speed a constant at both those figures to get maximum speed? or would we keep it between the 2 rpm readings?

Also, when upshifting, what would be more important, the highest torque or hp rpm? When getting off the mark, i understand torque would be more important and as a vehicle is in motion, it is hp which would keep it getting to higher speeds.
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Old 12th November 2009, 17:05   #113
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Originally Posted by Shan2nu View Post
Ok then, tell me what exactly you meant to say in this para.

Rpm definitely controls the horsepower, higher the rpm higher will be the power, the best example is a f1 car, a 3.0 NA V10 produces around 900+ Bhp @ around 18000rpm, you see here power output is 300Bhp/litre, without higher rpm these figures are impossible (even if you use turbos or superchargers).


Shan2nu
I meant to say that if the engine revves faster it makes more power, either if it is a NA or FI engine, the power output of all engines depends on what speed they are running, isn't?

Even in FI cars engines the power output is relative to rpm, what I am trying to say is a FI engine (example evo X's 290Hp @ 6800rpm) running at 6000rpm will be delivering more power when compared to the same engine running at 2000rpm, You see the rpm controls the power here too (Atleast upto its peak output, later it may fall slightly)
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Old 12th November 2009, 17:35   #114
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Quote:
Originally Posted by Shan2nu View Post

You can't make a 2ltr NA engine produce 200bhp@6000rpm but you can make a 2ltr FI engine do it.

A Veyron does 123bhp/ltr@6000rpm, try and find a NA engine which can do this, you wont.

145bhp/ltr@6800 is unheard of in the NA world but FI engines do it.


Shan2nu


Do you know anything about this engine?
4A-GE (20-valve) Fourth Generation "Silver-Top"

The fourth-generation 4A-GE engine was produced from 1991 to 1995.
It is a 1.6L NA engine and this engine produces 158 hp (118 kW) at 7400 rpm with 162 Nm (119 ftlbf) at 5200 rpm of torque.

It is a NA engine of early 90's technology..

(Veyron is powerful because it has got a bigger engine)

Everyday technology keeps advancing, today's latest will be soon future's scrap..
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Old 12th November 2009, 17:42   #115
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Originally Posted by Shan2nu View Post
An FI engine can produce lot more bhp at low revvs as compared to a NA engine.
I'm not even sure Naturally Aspirated (NA) engine and Fuel Injected (FI) are the right comparison. You can compare a NA to TC engine and Carburetted and FI engines. Please correct me if I'm wrong

Cheers
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Old 12th November 2009, 17:56   #116
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Originally Posted by thewhiteknight View Post
When upshifting, what would be more important, the highest torque or hp rpm? When getting off the mark, i understand torque would be more important and as a vehicle is in motion, it is hp which would keep it getting to higher speeds.
Lets take a ford fiesta (Indian), The 1.6 NA engine puts out
Power (PS@rpm) 101PS @6500rpm
Torque (Nm@rpm) 146Nm @3400rpm

First clarify whether you are upshifting (5-4-3-2-1) or downshifting (1-2-3-4-5)..

If you downshift at any rpm the rpm falls instantly due to response from the transmission..

Lets say you are changing a gear (1-2) at 3400rpm (max torque) the rpm will fall to say 2000rpm you have to again rev it harder to pull your vehicle
where as
when you shift at 6500rpm (max power) the rpm will fall to say 3500rpm, as you can see above as you downshift the rpm goes into max torque zone and you will be utilizing max torque (maybe slightly less) which in turn propels your ride even faster..

There is a lot of information on when to shift on TBHp in various threads, please go through it to understand it better..

Keep revving...
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Old 12th November 2009, 18:00   #117
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Originally Posted by MileCruncher View Post
I'm not even sure Naturally Aspirated (NA) engine and Fuel Injected (FI) are the right comparison. You can compare a NA to TC engine and Carburetted and FI engines. Please correct me if I'm wrong

Cheers
Here
NA == Naturally aspirated engines
FI == Forced induction engines (Turbo/Supercharged engines)

FI here doesn't means Fuel injected..

This should clarify your all doubts..
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Old 12th November 2009, 18:01   #118
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Originally Posted by MileCruncher View Post
I'm not even sure Naturally Aspirated (NA) engine and Fuel Injected (FI) are the right comparison. You can compare a NA to TC engine and Carburetted and FI engines. Please correct me if I'm wrong

Cheers
Here Shan2nu is talking about Forced Induction (FI) as opposed to Naturally Aspirated (NA) engines, not Fuel Injected. A turbocharged engine is an example of FI.
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Old 12th November 2009, 18:04   #119
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Originally Posted by watashi75 View Post
Here Shan2nu is talking about Forced Induction (FI) as opposed to Naturally Aspirated (NA) engines, not Fuel Injected. A turbocharged engine is an example of FI.
Is Forced Induction similar to Ram - Air intake?
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Old 12th November 2009, 18:07   #120
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RAM intake allows more air into the engine but Forced induction sends more air by compressing it through the same pipe,
You cannot get much benifits from RAM intakes when compared a small FI system..

Keep revving..
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