compression ratio, fuel and boost

[whysmees]

it is a thought that is playing on my mind?
pre detonation, fuel octane and forced induction.
a 10:1 compression ratio is pushing the limit of standard fuel and the allure of premium fuel is at hand, yet to be a LPG converted 11 or 12:1 is possible as i understand.
ok so i have 8:1 with cast iron heads to play with and LPG is a ok with me, decompression is of no interest and so the question is at hand regardless of the choice of engine i have to play with....
compression ratios to fuel and level of boost, is there a graph or method to get the optimum economy and power, using either a sc12 or 14 supe4rcharger on either a 1600 or 2000cc engine

[NME308]

Hi there,
The following is well worth the read and explains in rather great depth!

The original article found here:
Increasing CR vs boost pressure

Edit 10/19/07. To whom it may concern. The source of the material below is me.
Plagiarism = literary theft, or trying to pass someone else's words off as your own.
For myths on copyright, please see Brad's web page: => 10 Big Myths about copyright explained
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This question seems to come up time and time again. Is it better to increase the static CR or boost pressure. There are a couple reasons why supercharged or turbocharged engines run lower static compression ratios. A static CR in the range of 8-9 is very common. Here are a couple considerations.

Consideration #1
Heat from compression by a supercharger or turbo can be removed (for the most part) through use of an intercooler. Heat from compression within the cylinder cannot. Also, the cylinder pressure at the end of the compression stroke (prior to ignition) goes up exponentially with an increase in static compression ratio, versus a linear increase with boost pressure. Therefore, increasing the static CR is going to unavoidably push you closer to the knock limit for a given fuel. In other words, the octane requirement goes up more by increasing the static CR than it does by increasing boost.

For example, increasing the static CR from 8.5 to 9.5 increases the temperature within the cylinder at the end of the compression stroke (but before ignition) by ~63°F, (assuming IAT2 = 130°F and ideal adiabatic compression with γ = Cp/Cv = 1.4. I won’t bore anyone with equations. The situation doesn’t change much even if IAT2 were only, say, 100°F. In that case, the increase in temp at the end of the compression stroke goes up by ~60°F for the same increase in static CR). Also, the pressure at the end of the compression stroke (before ignition) goes up by ~97 psi from 574 psi to 671 psi, assuming atmospheric and boost pressures of 14.7 and 14 psi, respectively. On the other hand, increasing the boost pressure from 14 to 15 psi increases the outlet temp of the compressor by only ~11°F, assuming AE=60% and IAT1 = 90°F. And by further assuming an intercooler efficiency of 80%, the increase in IAT2 is only ~2°F. Hence, the increase in temp at the end of the compression stroke will hardly change at all. Also, the increase in cylinder pressure at the end of the compression stroke only goes up by ~20 psi (from 574 to 594 psia) with this increase in boost pressure.

So summarizing the effects of increased temp and pressure at the end of the compression stroke for the two cases:
Increased CR from 8.5 to 9.5: ΔT = ~63°F and ΔP = ~97 psi
Increased boost from 14 to 15 psi: ΔT = ~2.4°F and ΔP = ~20 psi

A higher temp and pressure increase the likelihood of deadly preignition for a given octane fuel. And for those astute observers that know the physics I’ve applied, yes, although I’ve idealized things to keep it simple, (by not including effects such as heat loss thru the cylinder walls during the compression stroke or ignition and valve timing in the calculations), I’m sure they’ll also recognize that this doesn’t change the conclusion.

Consideration #2
Power is increased by two completely different mechanisms for the two approaches. Increasing the static compression ratio increases power via an increase in thermal-conversion efficiency. Increasing boost pressure increases power via an increase in mass-air flow rate. There’s less gain in thermal-conversion efficiency (and hence power) via an increased static CR compared to the power gain by increasing the mass-air flow rate via an increase in boost pressure. For example, increasing the static CR from 8.5 to 9.5 results in an increase in thermal-conversion efficiency (for an ideal Otto cycle) of about 3.2%. On the other hand, increasing the boost pressure from just 14 psi to 15 psi, increases the mass-air flow rate by about 3.5%. If boost pressure is increased by 2 psi, (from 14 to 16 psi), the increase in mass-air flow rate will now be more than twice that compared to the increase in thermal-conversion efficiency, (~7% vs ~3.2%), and ΔT and ΔP still won’t be as great as they are when increasing the static CR from 8.5 to 9.5. Therefore, not only can it be “safer” from the knock point of view, but a little more power is gained as well, (relatively speaking that is).

In conclusion, I would contend that for a forced-induction application, that low compression is in general, the better way to go.
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Boost vs compression ratio Part II

Since writing part I, there have been some comments made that I felt warranted a part II. Comments such as, “Good stuff, but peak combustion pressure and temperatures are far higher than they are pre-ignition.” Or, “I have more gauges than a pimply faced teenager in his Honda Civic, so I can run closer to the ragged edge.” Or, “The engineers didn’t put 8.5:1 compression pistons in the Terminator, or 8.4:1 in the FGT & GT500 for any thermodynamic reasons.” Or the classic, “It’s all in the tune.” Although most of these statements are not totally without substance, the basic conclusion is unchanged. Even though adjusting timing and AFR can reduce the tendency to knock, and although peak combustion pressure and temperatures post ignition are significantly higher than pre ignition, or in spite of how close one cares to run to the knock limit of a given octane fuel, (for a given fuel and AFR, etc.), the engine can make more power by reducing CR and increasing boost pressure, than the other way around for the same peak cylinder pressure. This is why it is common to see lower compression ratios on SI forced-induction motors. Obviously there are tradeoffs, however, as a direct result of the lower thermal-conversion efficiency. But when it comes to maximizing power and torque output at wide-open throttle for a given octane fuel, etc., lowering CR and raising boost pressure is the safer approach.

This conclusion is based primarily on two basic facts:1.Mean-effective pressure (MEP) goes in direct proportion to the mass of air ingested, (which is directly related to boost pressure), but goes up “sublinearly” with compression ratio, CR
2.To good approximation, peak cylinder pressure goes in direct proportion to both.
And as mentioned in Part I, while heat from compression by a supercharger can be effectively removed through use of an intercooler, heat from compression within the cylinder cannot. As a result, peak combustion temperatures do not tend to rise significantly with increases in boost pressure, whereas they will go up with increased compression ratio. And as we all know, a lower peak combustion temp also reduces the likelihood of knock. One also needs to recall that power and torque are directly proportional to MEP, where the mean-effective pressure is defined as the “effective” pressure over the cycle, which is equal to the work generated over the cycle divided by the displaced volume. In other words, if the indicated MEP goes up X%, indicated power and torque will also go up X% at a given engine speed. Additionally, one needs to recognize that for a given octane fuel, AFR, etc., that as peak cylinder pressure and temperature are raised, eventually the engine going to knock, or detonate. This shouldn’t be any surprise since this is exactly how a fuel’s octane, (i.e. its resistance to knock), is measured. It is put in a special test engine whose compression is raised until the engine knocks. (The reference fuels used for comparison are iso-octane defined as having ON = 100, and normal heptane having ON = 0.) References: Octane rating - Wikipedia, the free encyclopedia. Or section 6 here => http://blizzard.rwic.und.edu/~nordli.../gasoline.html

For the ideal Otto cycle, it is very easy to derive expressions for the peak combustion pressure (P3) and temperature (T3), and mean-effective pressure. As in Part I, I’m not going to derive or show all the math, but simply get to the bottom line and show the results. For those that want the details, the interested reader is referred to any number of good text books on engine fundamentals, (Taylor’s, Heywood’s, etc.). I’ve also posted the equations, sans proof, in this thread: http://www.eng-tips.com/viewthread.c...=215499&page=1

Taking the ratio of mean-effective pressure to peak cylinder pressure, or vice versa, one will find that the dependency on boost pressure drops out and the ratio only depends on CR for a given fuel and AFR. (Note - timing does not factor in simply because MBTT at TDC for the ideal cycle, but this does not change the conclusion.)



where the thermal-conversion efficiency for the ideal cycle is given by, ηt = 1 – CR^(1-γ), cv is the constant-volume specific heat for the mixture, ηc is the combustion efficiency, Qhv is the fuel’s heating value, and γ is the polytropic exponent which can be taken to have a value of around 1.25-1.3, over the full cycle, (Ref., H.M. Cheung and J.B. Heywood, SAE paper 932749).

Using these results, one can plot the ratio of indicated mean-effective pressure to peak cylinder pressure, iMEP/P3, vs compression ratio. From this plot, one will see that as CR goes up, the ratio iMEP/P3 will go down. (See plot below).



What does this mean? It means for any given maximum tolerable peak pressure (for a given octane and AFR), that iMEP will be higher at a lower CR than at a higher CR, independent of boost pressure. Said another way, this means peak cylinder pressure climbs faster than indicated mean-effective pressure does as CR is increased, whereas both P3 and iMEP will climb at the same rate with boost pressure. Therefore, for any given fuel and AFR, etc., one can make more power & torque at any given engine speed by reducing CR and increasing boost pressure, than the other way around for the same peak cylinder pressure. The tradeoff is a lower thermal-conversion efficiency, which translates to a higher specific fuel consumption (pounds per hour per horsepower) and a “doggier” response at part throttle.

Although the above conclusion was based on analysis of the ideal cycle, a more complete thermodynamic model including finite burn duration, heat loss, spark timing, etc, will show the same trends and lead one to the same conclusion. As an example, cylinder pressure and temperatures vs crank angle for two engines with the same iMEP, but different CR and boost pressures are shown below. As can be seen, the engine with the higher CR has higher combustion pressures and temperatures, making it more likely to knock for a given octane fuel.




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Boost vs Compression Part III – Measured data

From posts in another related thread:

Measured data, example 1, turbocharged Cobra:
Let’s use the publicly available, actual data as posted on this web site. Using a turbo Cobra as an example, consider the data in the Terminator Summary of turbo data thread. (Link => Summary of turbo data)

From this data, the "typical" turbo Cobra running 15 psi makes around 670 rwHP, (with stock displacement and CR.)

Now say one increases CR to 10:1 from 8.5:1, and pulls a few deg of timing to avoid knock.
How? Click link => Spark timing impact on knock

Say this increase in CR nets a 6% gain as published in a recent magazine article. This then translates to 1.06*670 = 710 rwHP. So yes... a nice gain of 40 rwHP.

But now lets keep timing at this same reduced amount, but instead of increasing CR, increase boost until one gets to the same peak cylinder pressure & temp. So how much boost is added? Use the well-known, effective-compression ratio as given by,

CReff = CR(1 + Pboost/Patm)

which comes about from how peak cyl pressure scales with both boost and CR. (Note – it also can take into account valve timing, by using the dynamic-compression ratio for the value of CR.) As Ed (eschaider) has pointed out, this effective-compression ratio is what VP (and others) uses to "rate" their various fuels.

If one goes through the math, they’ll find that this would mean running around 20 psi at this same reduced timing, but with stock CR of 8.5:1, instead of 10.0:1. (Sorry...yes...a little math. The “prime” ( ' ) symbols indicate the “new” values of a quantity.)

Pb' = [(CR/CR')(1 + Pboost/Patm) -1]Patm = [(10/8.5)(1 + 15/14.7) -1]14.7 = 20.2 psi

Again from the regression analysis of actual data shown in the turbo data summary thread, (link provided above), the change in power with boost is roughly 22 rwHP/psi. So this means with the same reduced timing, the CR=8.5 motor with 20 psi would now be making 670 + 22*(20 - 15) = 780 rwHP. So a gain of 110 rwHP.

Example from measured Turbo data - summary:
Baseline case; CR = 8.5:1, boost = 15 psi, ~670 rwHP
Higher CR; CR = 10:1, boost = 15 psi, ~710 rwHP
Higher boost; CR = 8.5, boost = 20 psi, ~780 rwHP

Measured data, example 2, twin-screw KB Cobra:
Here’s a similar example of the same analysis for a blower car from chassis-dyno measurements made on the same car. (I didn’t have data at 15 and 20 psi, but I do at 17 and 23. Using the effective-compression formula given above, the higher boost would come out to 22.6 psi for this case, but I think you’ll give me the 0.4 psi.)

For this analysis, the data is taken from the same vehicle, on back-to-back pulls, on the same dynamometer, and covers the full rpm range – not just peak power. Since it is from actual measured data, it includes all additional effects such as any potential belt slip, increased SC drive power, etc. Data for the CR=10:1, 17 psi case came by scaling the CR=8.5, 17 psi case up by 6%. (A worked example of how power scales with CR, including actual measured data, is given later in this thread in post #23, Link => Post #23.)



As one can clearly see, the higher-boost/lower-CR makes significantly more torque than that of the higher-CR/lower-boost case, all across the rpm range.

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Boost vs Compression Part IV – Impact of valve timing

From thread in 2011 Mustang Forum. Click link => Boosted Coyote Engines using VVT

Yes, valve timing also has an impact and can reduce the risk of knock. One can delay the inlet-valve closing (IVC) event, which reduces the so-called dynamic compression ratio. Basically, when you delay the IVC, you aren't running as high a (dynamic) compression ratio anymore which as explained above, helps reduce peak cylinder pressure & temp, thereby reducing the risk of knock. Below is an example for the 5L Coyote engine in the new 2011 Mustang GT.



Therefore, the engine calibrators can make use of the variable cam timing on the 5L Coyote engine to reduce its relatively high-compression ratio, to a lower compression ratio when using forced induction.

In your case you will be realistically boost limited by the supercharger and its efficiency range. This means you will need to look at what fuel you intend to run vs what boost your charger of choice can produce on the cubic capacity of the engine you end up running vs what power you are ultimatly chasing and making the required compromises from there...

I run 7.0:1 compression and am pushing 22psi boost comfortably on 95 octane fuel. There is still plenty of room to wind it up yet. On the other hand a blower of the toyota oem variety would likely not provide enough boost comfortably to make such low compression ideal.

I feel that at around 8.something to one compression as you propose with one of them blowers (sc12 or 14) on one of your engines (1.6 or 2.0 liter) you should be able to run decent amounts of timing while running the blower within its acceptable limits and obtain very worthwhile power. Of note in other research by the Honda F1 engineers during the turbo era - once boost limits of 2.5bar were introduced - was their finding that keeping 30 or so degrees timing in the engine was far more beneficial than upping the compression further than 9.5:1.

Cheers and good luck!
Jason

[Duk]

Consideration #1
Heat from compression by a supercharger or turbo can be removed (for the most part) through use of an intercooler. Heat from compression within the cylinder cannot.

That whole post is excellent! But I feel that the above comment is a little bit misleading or at least should be explored a little further.

At the risk of dragging the thread off topic, the most effective part of water injection is that is absorbs most of the heat that it can absorb, within the combustion chamber.
That is, most, if not all water injection systems could never hope to fully evaporate all of the water they inject into the post intercooler air stream (unless they injected a very small amount and the intercooler was performing very poorly or is worked incredibly hard).
Autospeed Article has some excellent reading on the subject.

[fixeruperer]

piston top and combustion chamber design also come into play as well, also cam timing. some engines like comp,timing and boost and some just dont, but with a bit of tweaking to the chambers you can change that.

i run 9.4:1, 21psi boost and bp98. happily and im going to crank it to 25 when my new turbo comes in.

sc12/14 are pretty inefficient blowers, they run too hot.

[Duk]

piston top and combustion chamber design also come into play as well, also cam timing. some engines like comp,timing and boost and some just dont, but with a bit of tweaking to the chambers you can change that.

i run 9.4:1, 21psi boost and bp98. happily and im going to crank it to 25 when my new turbo comes in.

What sort of full boost, peak torque ignition timing are you using? Being able to pull heaps of ignition advance out will help keep the engine alive (no detonation), but exhaust gas temperatures go to the moon when you do. For short squirts on the street or drag racing are 1 thing, but sustained minute after minute high boost and really low ignition advance temperature related issues are another.

sc12/14 are pretty inefficient blowers, they run too hot.

And the rest. They take to much power to drive and have to much internal leakage (part of the reason why the have higher discharge temperatures. The other sign is the fact that a 1.2 litre blower has 3 times the capacity of 1 of the cylinders of a 4AGZE but struggles to get 1 bar of boost into the engine).

[banana_socks]

That's a great explanation on the subject, might have to copy down the link. However I think there is one major thing he overlooked: whether you are actually producing that much boost. It might be beneficial to lower your compression ratio and run 25psi, but what turbo is going to be able to supply that from idle through to redline? Whereas a high compression ratio has the same benefits at any rpm.
Just like anything its a compromise. Lower CR and boost might make more power, but it won't be as streetable.

[whysmees]

it is a 1962 toyopet stout and one of the first toyota's in australia on a 1974 toyota stout chassis


i doubt the the motor i am pondering is up for hi performance, although i feel a supercharger pushing low boost would be worth a try as i have found no info on anyone doing anything with this style of engine apart from removing them.
the motor is a 5R 106hp (timing chain version ) the timing gear version cracked heads, stripped timing gears and was more the problem child. but i have a timing chain version that has done over 900,000kms untouched apart from a head gasket and i have four spare 5R engines in the shed.
as i live in queensland i am not allowed to fit an engine that is more than 10% bigger in cc or hp than factory options as there is no engine mod codes (blue plate) for stouts that allow a 6 or 8 and so am left to play with what i have.
the most common replacement of the 5R was a holden six and at one point the original chassis had been murdered in such a manner, as a stouts engine bay ain't that big.
my goal would be to one day to remove the 6.1:1 diff centre and fit a 4.1:1 out of an early cruiser as they are the same design, instead of fitting the later dyna trucks 5 speed O/D, which to me seems backward.
a supercharger over turbo dose not have lag and the extra power would be needed from idle "to avoid forklift syndrome", all i would like to do is get the 5R stock 2000cc on LPG "so the increased heat is not such an issue" to do 80km/h + with ease....if possible

[Duk]

It's fair enough that you want/need to work with what you have.
My biggest suggestion to you is that you have a good programmable ignition timing controller. That way you can have the benefits of both on boost safety and performance and off boost response and economy.
http://www.autosportlabs.net/Megajolt_Lite_Jr. that works with the Ford EDIS system looks pretty good, even though it uses 'only' a 10x10 map. It does have an "Analog auxiliary input to further correct ignition map", you could use an inlet air temperature sensor to help with changes in inlet air temperature (I recently took my Adaptronic E420c run, twin charged AW11 MR2 through a hilly road for the second time, only this time the air temperature was up 10-12*. Without the inlet air temp. sensor fitted to utilize the ignition retard with higher temps., there was audible detonation when on boost in 4th gear (steep hill) where before I could be on boost in 5th gear without audible detonation).

[whysmees]

i would like to keep it as basic as possible and a bit old school, my thought is a little lever in the cab to advance or retard the timing as i wish or need.
solid shaft distributors is what i cut my teeth on in the 800cc lj80 suzuki 4x4 and to be able to alter the timing manually while on the go is what i seek.

[whysmees]

Wood Gas Engines
Low-tech Magazine: Wood gas vehicles: firewood in the fuel tank

[boxh34d]

NME308 - that is pretty good lot of info!!!

i would like to keep it as basic as possible and a bit old school, my thought is a little lever in the cab to advance or retard the timing as i wish or need.
solid shaft distributors is what i cut my teeth on in the 800cc lj80 suzuki 4x4 and to be able to alter the timing manually while on the go is what i seek.

Ignition timing for a supercharged engine is a whole lot easier than for a turbo. A turbo can produce varying boost at a static RPM and throttle position due to load. (and any vice verce depending on what circumstance is present). A blower on the other hand produces a pretty much fixed pressure regardless of RPM and load due to it's positive dispacement nature. ie, if you have a 2L engine, and run 0.5 bar of boost via a blower, it will be an effective 3L engine. It will still havae a torque curve similar to the original engine, and require a timing curve similar. It will also require adjustments to be made for the increased temperature of intake air, and drive requirement of blower, but it is still going to follow pretty much the same power and ignition curves as the naturally aspirated engine. I would say that the standard engine is pretty lame in the timing department anyway, so it'll probably be fine for a few psi of boosted air.

Whilst most will say a roots blower is hugely inefficient, it is not always the case. A roots blower was designed for one thing - to move large, consistant volumes of air at low pressure. At low pressure, and (reletively) low speeds, they can be quite efficient - up to almost 90% in industrial applications, and have almost 0 leakage. Once the pressures and rpm start rising, then drive power increases sharply, as does leakage, which is quite different to twin screws which have leakage at lower rpm and boost). Most roots blowers in a automotive application are capable of greater than 60% efficiency as long as they stick to what they are good at - lower pressure and moderate speed for the blowers displacement
In the case of an SC14, at 5psi, and flowing around 400 M3/hr, it's around 65% efficient, will take around 6kw to drive, with a temp increase of 40 to 50 deg C. I would say this should be enough for around 140HP at the engine in the case of your pretty ordinary 5R.

I would be inclined to run your 8:1 compression with a bit of boost through a toyota blower and enjoy it. Throw a larger carby such as a 2 barrel off a red holden (around 2.8 to 3.3L) etc on the intake side of it, and this will go a long way to keeping the charge cool, and help increasing the blower seal to increase efficiency. Throw it on the dyno and tune the carb and spark, and be happy.

-----Edit-----
I've just come off call and the first few didn't touch the sides. . . . so hopefully that makes sense.