Engine efficiency is directly related to compression ratio, I would like to see what you guys think are the reasons behind this.
I am working with some thermal efficiency 'stuff' and would like to know why a high compression ratio allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.

There are a few theories out there, but I would like to see what the brains of k20a.org can come up with
Thanks for your time guys

youre working with thermal efficiency "stuff" but dont know why a higher compression ratio engine is more efficient than a lower compression one??? put down the tools now, and start reading books. its not hard to understand.

 

then explain?

 

i'm a mechanical engineer...... I work at Honda's Anna Engine Plant, and have since april of 2004. i dont need to explain. I laugh because its common sense. I laugh even harder because I knew the answer to this far before I went to college.

 

Dude, what is your problem? I'm aware of the reasons, im just trying to start an informative thread, so that people don't understand, can have a good read.

It seems like you are just dying to let people know just how clever you are, by laughing at people who don't share the same amount of knowledge as you.
Thats great buddy, thanks for your imput.

 

bump...i want to know 2

 

bump i would like to know ?????/??????

 

Ok guys, here is my basic understanding of this, please feel free to add to this...

To understand engine efficiency and how it relates to compression ratio, a look at the correlation between thermal efficiency and the mechanic energy achieved from a given mass of air/fuel mixture.

A higher compression ratio will promote a more efficient burn of the air/fuel mixture, due to the oxygen and fuel molecules being closer together.

E.g. for an engine with a combustion chamber volume of 105cc and a total cylinder volume of 1000cc the static compression ratio will be 1000:105 or 9.5238:1 (9.3:1)

Air/fuel mixture (drawn in by the negative pressure caused by the piston moving down the cylinder), assuming there are no variables, will be proportional to the total volume of the cylinder, the value of the mass of the air/fuel mixture in this case will be referred to as X
Another engine, with a combustion chamber volume of 100cc and a total cylinder volume of 1000cc, will have a compression ratio of 1000:100 or 10:1
Assuming there are no variables, the total mass of the air fuel mixture drawn into the cylinder, will still be X

The air/fuel mixture in the higher CR engine is being forced into a smaller space, causing the oxygen and fuel molecules to be closer together. This promotes better mixing of the air/fuel and as the air increases with heat due to the compression, it will also result in improved evaporation of the fuel.

Fuel molecules that are more concentrated, will result in increased expansion of gasses, and will produce more energy when ignited.

So, given a value for the air/fuel mixture (X), a higher compression engine will extract more energy from it, thus making it more efficient.

 

100% Fail.

Well, again, as far as my knowledge goes, it has alot to do with fuels and octane ratings. You are always wise to choose your octane rating based on effective compression ratios.

With a forced induction engine, it is especially wise to choose octane based off effective CR. Any form of F/I will effectively raise the CR of an engine.
Using the formula

((boost PSI / 14.7) +1 x static CR = effective CR

The effective CR of the engine is now significantly higher than the static CR, as the addition air mass has been taken into account from the forced induction.
If the octane was chosen based off the static CR in this case, it would not provide the sufficient resistance to detonation required to allow the engine to run safely.

You would either have to lower the static CR, lower the effective CR by valve timing or run a higher octane fuel. Its usually more feasable to lower the static CR.

Again, please add to this guys, just want to see if there any other thoughts on the matter

101% Fail






Look at a chart of a carnot cycle. All parts of the engines cycles only absorb/rob/steal energy from the engine except for one. Expansion. The amount of energy you convert from gas pressure to mechanical torque on the crankshaft depends exactly on the amount of expansion you are able to perform on this gas mixture. This expansion is the only way an engine is able to extract energy from the fuel/air mixture you've burned. The combusted mixture has potential energy stored in the form of pressure and heat. As it expands, this mixture under goes a cooling as the volume increases. The amount that the temp is decreased by, and the volume is increased by directly relates to the amount of energy converted to mechanical energy at the crankshaft. The great the expansion ratio, the greater the amount the engine can cool the mixture, and hence the greater amount of power can be extracted.

You can use cam profiles with very late IVC events (or very early IVC) to create an imbalance between dynamic compression ratio vs expansion ratio. This is called a "miller cycle" engine. It is able to lessen compressive losses while keeping high expansion power extraction from the mixture. It improves economy at the cost of performance, as it functions as though its a smaller displacement engine than it physically measures. They are often supercharged to recover this loss of power, and a positive displacement blower acts as a check valve to harness the returned intake pulses from the 30-40deg late IVC events.

 

How bout throwing in why having a lower compression is better when your running forced induction and how that works question. I'll leave the explanation for someone smarter than me.

 

Well, again, as far as my knowledge goes, it has alot to do with fuels and octane ratings. You are always wise to choose your octane rating based on effective compression ratios.

With a forced induction engine, it is especially wise to choose octane based off effective CR. Any form of F/I will effectively raise the CR of an engine.
Using the formula

((boost PSI / 14.7) +1 x static CR = effective CR

The effective CR of the engine is now significantly higher than the static CR, as the addition air mass has been taken into account from the forced induction.
If the octane was chosen based off the static CR in this case, it would not provide the sufficient resistance to detonation required to allow the engine to run safely.

You would either have to lower the static CR, lower the effective CR by valve timing or run a higher octane fuel. Its usually more feasable to lower the static CR.

Again, please add to this guys, just want to see if there any other thoughts on the matter

 

isnt it basically the higher the compression the more squished the air and fuel become making it more volatile once ignited...since the mixture is so compressed, when it is then ignited, it is forced down the cylinder wall with greater force than one with a less compressed mixture...thats the way i see it..lol

 

whoow.. slow down college.. i cant keep up with all that technical jargon..:silly: But yeah..in a nutshell thats about right. I didnt even know there was a formula for that. I would imagine that the more compression there is the more explosive the energy it would create. Sounds good to me anyway

 

laymans terms always work best when your not dealing with scientists or engineers. lol@squished.

 

We don't make fun of people that want to learn and we give everyone the benefit of the doubt. Maybe you should do the same.

 

If you're not here to be of any positive use to the thread, then dont post on it.

That goes for any thread on this forum...

 

agreed. why i love this site. seems to be a much higher concentration of adults per say than any other site i frequent.

 

In a nut shell, the reason why compression ratio affect the engine efficiency is as below:

although we are keeping talking about 12.5, 10.5, 13.5 CR, what we are refering to is actually the static compression.

But.... when the engine is running, the static compression fly out of the windows. it does not applies anymore.

The problem is that static (advertised) compression ratio assumes that you have a cylinder full of fuel mix at bottom dead center and all that fuel mix is compressed into the combustion chamber at top dead center. The fact is that you really don't make serious compression until the intake valve closes and seals the air fuel mix in the cylinder. The actual running compression is often considerably less than advertised static compression.

With cams and the engine running, not to mention VTC, our valve are not totally closed at BDC before the crankshaft move and compress the air.

why does i mention all this when we are talking about Engine efficiency?

As an example, consider a 10:1 Car with a 3.48 stroke 6" rod and very hot cam that closes the intake at 90 degrees after bottom dead center. This engine will think it is running with 6.17:1 compression pistons and will be happy with 80 octane fuel.

As a general rule, the best available pump gas will work with 8:1 effective compression ratio.

To get 8:1 effective compression ratio with the above rod, stroke, and cam intake closing event, you would need a 13.2:1 static compression piston.

A major part of current engine R&D centers on designs that allow higher compression pressures with reduced octane fuel. The effective compression calculator puts all engines on a level playing field.

We say that 8:1 effective compression is about it for pump gas. Most flat head engines will not work well at 8:1, but some advanced design engines will surpass the 8:1 rule.

Why are we so concern about all this?

its because compression ratio affect the thermodynamic efficiency of the burning.
Typically, the higher the ratio, the more complete the fuel burns. the more complete the fuel burn... more power..

Now that why higher static compression increase the engine efficiency.. But only up to a certain level. break the rule... = break ur piggy bank for a new engine..

Engine efficiency also have 2 more factors: namely, mechanical and air flow.

 

ok im lost lol so what is the big difference between static compression and dynamic compression?? and if static compression is what your pistions are rated at out the box why dont alot of tuners like to tune more then 12.7.1 on pump gas ???

another question is if cam size and angle effect compression then what is being said is that a very big cam and lets say 13.7:1 compression can in fact be tuned on 93 pump gas.. is this true or no??

 

well.. I don't really recommend it..

U have to retard your ignition to lower the dynamic compression at the time of ignition to 8.1 and below.. using higher ron, u can advance it more due to knock resistance of higher RON to about 8.5 maybe?

For me, I always use the highest RON rate oil... for my piece of mind... LOL..

and U got the wrong idea about cams...

what cams you wanna use is dependant on what type of power profile you want.. of course, they must have improvement.

But 1 thing is clear, high lift and duration cams are good at working at high rpms ranges .. and the higher the static compression, the higher the dynamic compression is at high rpm... = more power.. that provided your engine can rev that high..

 

well my end goal is around 315whp on pump... i want to send the head out and get it ported and run itbs.. the highest gas we have here is 93 out the pump and 2 places within 20 miles that have e85 witch is what i want to run being that its is going to b a road race car/ red light to red light toy every now and again

 

Wow. Interesting to see how one pompous idiot can make us all look bad. I am a mechanical engineering student, and think that the guy at Honda has his head too far up his ass.

The thermal efficiency of a gas engine is proportional to the amount of energy put into an engine, and the amount of work put out. Starting with the governing thermodynamic equation, [Net Energy - Net Work = internal energy], we can manipulate it to give us the efficiency in terms of compressions ratio.

The equation we use is EFFICIENCY = 1 - (1/Rc^0.4).
Rc - compression ratio

eg. Using an Rc = 11.1, we find that the thermal efficiency is:
Efficiency = 1 - (1/11.1^0.4) = 1 - (0.38183) = 0.61817*100 = 61.8%

This will give you a % for IDEAL thermal efficiency, neglecting all frictional losses and based on a few assumptions (ideal case, as is most engineering).

As you may be able to see from the relationship, as Rc gets bigger, the efficiency will increase, but will taper off after a certain point, at which the engine will approach it maximum attainable efficiency, also known as the Carnot efficiency. This is found by taking the hottest temp (combustion) and the lowest temp (atmospheric air), and using the following relation to find the max thermal efficiency of ANY HEAT ENGINE (gas, diesel, jet, steam, etc).

Carnot Efficiency = 1- (Tlow/Thigh) <--- MAX EFFICIENCY FOR ANY HEAT ENGINE.

Disclaimer: All temps should be done in base units, K or R (Kelvin or Rankine). Also, this is to analyze an ideal system only.

-Pat

 

Just wanted to clarify since I have not taken a thermodynamics class, out of 100% thermal efficiency, 61.8% of the thermal energy does work on the piston?

 

The 61.8% represents how much energy is "usable" from the entire amount of energy produced by the combustion, if you have a perfect air/fuel ratio. If you look at how much energy is produced from the engine per second (Joules/s), then you have your metric base units for Power (Watt = Joule/S), which can be easily converted to a horsepower value. Getting sidetracked...

In other words, if you put 1L of gas into an engine, you're engine is able to use 61.8% of that gas to make power on the piston, the rest of the energy goes to heating your engine up, hence all the cooling systems in a car's engine bay. My teacher used to say "... you could basically fill your car up 61.8% with gas and pour the rest on the ground and light it on fire, because that's all the good it's doing our engine." Again, restating that we can only get as much power out of our engine as our efficiency will allow us.

So, yes, the 61.8% is what IDEALLY is being used to push on the pistons, but in reality you get less than that. Help at all? It's not exactly applied mechanics, but it gives you an idea of what the guys think about when designing engines.

 

Got it. Thanks. :up:

 

Here's my take, which touches upon the main purpose of compression in the first place.

In order to burn something, anything that isn't already burning that is, you need to input some amount of activation energy in order to begin the combustion process. While the spark is the final trigger in a conventional motor, the compression stroke inputs energy into the air/fuel mixture by heating it. Recall that compressing any gas will raise its temperature. This is why compression is necessary in a motor. It allows the air/fuel mix to be ignited quickly and uniformly. Without compression, the air/fuel mix may still burn, but it will be a much slower burn and the mix will not ignite as uniformly.

So what benefits are there to increasing compression? Increasing compresison increases the heating of the mixture, making it even more volatile and ignitable. This results in a faster burn. The more rapid the expansion of the gas, the more power that you make. There are obvious limiting factors, such as the auto-ignition threshold of the fuel in use under running conditions. One of the main benefits of of the faster combustion event is that you can sustain power production into higher RPM. In layman's terms, the gas has to expand faster than the downward movement of the piston. As your revs climb, the piston speed naturally increases, which means there comes a limiting point when the combustion gasses simply cannot expand fast enough. The piston, in a way, begins to outrun the burning gasses. Higher compression generates a faster burn, which raises the RPM threshold at which this happens.

This also gets into why high compression motors can experience a lot of knock at low RPM. At low RPM, the spark must be delayed so that peak cylinder pressures do not occur before the desired crank angle. Because the a/f mix is lit off more uniformly, resulting in a faster burn, the peak pressure will occur sooner than on a low compression motor. High compression motors will always require less ignition advance at low RPM versus a low compression motor, all else being similar.

 

hi by skimming the cylinder head it increases compression but how mutch can you remove before the valve hits the piston

 

You are comparing apples and oranges now... I'm talking about all things, other then compression, being equal.

The thread is simply talking about compression/ cylinder pressure and it's effect on horsepower, not it's limits for a given enviroment.

 

I am just add food for thought for the Statement that Lower compression is better for boost, because you shot it down so quickly. I understand in the context of this thread you are correct.

 

I dont know if you can use this as an analogy but when you raise the pressure of something that is being combusted the power from that combustion is greater when the pressure is higher and lower when the pressure is lower.


say atmospheric pressure was 30psi on earth instead of 14.7 psi now. if a bomb goes off when the pressure on earth is 30psi that bomb is more powerful then when the ATM pressure is 14.7 psi. I think there is a law that states this more clearly.

 

i think this is the graph that people usally post up when this question is asked.

 

Great thread

And this image shows one of the many reasons why diesel engines are so much more efficient than the otto/petrol engine.

 

Thermal efficiency also depends on what would you consider Thermally efficient? If you consider a naturally aspirated motor only breathing about 1/4th of it's capacity each natural stroke then yes it's efficient In your perspective.
By the way this is true for all of our stock motors...

There are definitely ways to improve...Larger valve's, Porting, Honda engineered Vtec which is amazing in regards to how it increases the Volumetric Efficiency, and also TE. Nothing is better at increasing thermal efficiency in a normal piston engine than Boost(supercharger, Turbocharger), as most know today.

If I had to choose though, definitely a Turbo. Superchargers rob from the crank directly. So they only make about 2/3rds of the potential energy they are capable of. Unless they become independently driven someday? Instantaneous psi on the crank with no spool time 8O...Turbo has proven much more potential boost though. So turbo it is for me. Just studying the different factors that correlate to specific mod's and builds so that I may choose a nice set up.

 

thats why we buy hondata and tune with that

 

I know I'm new and this is a KA forum but im having some questions of my own after reading this thread.

Im running a F22a4 block with a F22b1 head
CR is 11:1
Bisimoto nitrous duty parts including valves, springs, retainers, camshaft etc etc.
Head has been ported
P28 ecu tuned on Crome

Lets say I didn't want to throw nitrous in it and wanted to go turbo.
Most people tell me that, its impossible b.c of my compression. They say I would have to run race gas only, or get a completely spot on tune.
But after reading, I'm not sure if that's right or not. Input?

 

You could boost on 93 or 91 but you'd need a good tuner cause the tq curve would be crazy

 

Well the guy who did the tune, tuned it for all motor. Hes regarded as one of the best tuners is missouri. But im sure about the quality of Crome.