Compression Ratio - Normal Aspirated Alcohol Injection

[AlkyUser]

I am in the process of assembling the following engine for my 68 camaro and would like to know my compression limit with alcohol.

I want to run About 50 GPH of straight meth at the peak. I have a chip burner and a wide band O2 and will zero out the power enrichment on the chip and be putting a 10% rich (lamba) curve of extra fuel using methanol under load (0 - 15" vacum). At cruise speeds there will be no alcohol injection (high vacum).

I will only be running regular unleaded (87 octane) fuel.

This is a every day street driven car. I am trying to build a high efecency motor.

The combo is,,,,


1987 5.7 Tuned Port Injection

Flat top pistons

50 cc Rhs IRON heads

Stock Roller Cam 204 /208 @.05" .430" / .460" (including the 1.6 rockers)

.035 Quench distance

I am getting down to putting the heads on.

With all my chamber and piston polishing done, I will have a measured 12 to 1 static compresion ratio.

If I take my heads into work and surface grind the heads .060"
I can have up to 14 to 1, or anything between.

Do you think 12 to 1 should be my limit, or for a little work and no extra money should I go up in compression.

 

[globi5]

Both methanol and ethanol have an octane rating of over 110 and they also have a higher latent heat of evaporation than gasoline. In addition, the air to fuel ratio is significantly higher (= more fuel needs to evaporate = less heat at TDC).

And apparently EPA managed to run an ethanol based engine with a compression ratio of 19.5.

http://www.hybridcars.com/component...blog_task,blog_view/joomblog_contentid,12019/

So at least my guess is, that running a compression ratio of 14 is save as far as detonation goes, but the question would probably be whether the engine is strong enough to handle that extra pressure. (Just my $0.02.)

 

[MikeHalloran]

Given the finite probability that no methanol will flow when it's demanded, I wouldn't go higher than the 12:1 you already have.

Be sure to assemble the methanol plumbing with large bore quick- connects and large radius elbows, so you can easily and quickly rod out the 'cheese'.




Mike Halloran
Pembroke Pines, FL, USA

 

[MWPC]

Be careful not to go too high or it will rattle on warm restarts with the 87 octane. The MAP is high during cranking. Your cam is pretty mild and will not bleed off much compression. If it rattles look for a cam with more intake duration or wider LSA. Good Luck. Ken

 

[AlkyUser]

Thanks for the input,

The rotating assemble I purchased is ratted for 500 hp and 6500rpm.

The cranking pressure will be close to 250 at 12 to 1 and over 250 at 13 to one. This is a educated guess as it was 170 psi at 9.4 to one. And yes 87 octane 9.4 to 1, .075" quench ( yes it was big) and agressive timing curve it rattled. On the hottest day it would rattel with 94 octane.

I detonated the crap out of the engine before I tore it down, I learned alot sbout detonation at rpm and map, also learned about the efects of power enrichment - it actual induced detonation.

I found no engine damage from all the heavy detonation that it experenced.

Now with increased cranking pressure and higher combustion chamber pressure I will be avoiding detonation as much as possible.

Yes I will be pushing the engine, but no more then a supercharged engine wiht about 6psi boost.

I am still not sure about what compresion ratio I should go with.

I would like to reask the question.

If I was building a engine Using 104 Octane race fuel, what would be the recomended compresion ratio be? Fuel injected with detail to assemble, short duration cam.

There is alot more experence in that area I would think.

 

[AlkyUser]

"Be sure to assemble the methanol plumbing with large bore quick- connects and large radius elbows, so you can easily and quickly rod out the 'cheese'"

Has there been blockage problems asociated with meth?

 

[AlkyUser]

I have never heard of detonation when cranking, but there is a table I could play with that would help fix that problem in the chip.

I can not copy it, but it basically has 5 degrees of timing on crank up to 180f then it cranks at 0 above 180f. I believe I could put negetive #'s in there.

 

[AlkyUser]

I would like to edit a mistake from the peginning of the thread. I said 50gph - I meant 50LBS Per Hour

 

[MikeHalloran]

I couldn't find a direct reference, but ISTR that Indy car crews routinely drain (and disassemble to the extent necessary) the entire fuel system right after a race so that the methanol won't clog it up with a whitish goo they call 'cheese'.

Something similar is discussed briefly toward the bottom of this page:

http://yarchive.net/car/methanol.html

... which includes a lot of conflicting information and assertions about whether water is or is not desirable, and on whether methanol does or does not eat aluminum.

Of course, the net is chock full of references to direct methanol fuel cells and how magical they are going to be, once the mysterious clogging problems are solved...



Mike Halloran
Pembroke Pines, FL, USA

 

[boatmotorgod]

I ran some numbers through an engine dyno program. At 12:1 you should see over 250 psi cranking, over 300 psi at 14:1. I would hate to see over 220 on race gas. I assumed 114 lobe centers, a factory cam could be as high as 118. I think its going to ping like crazy. You'll have to raise intake duration to something like 222 at 12:1 to bleed the compression off to below 220. If you maintain wide lobe centers, you will minimize reversion at high vacume crusing conditions. But the fuel economy won't be as good as with the smaller cam. To run the smaller cam you will have to lower the C/R to around 10.8:1. Actually the larger cam wouldn't be as detonation prone, and would make more power.

 

[globi5]

Although this is not a direct injection application, it is nevertheless interesting to note, that Scania increased compression ratio of their diesel engines from 18 to 28 in order to run them with E95.

http://www.greencarcongress.com/2006/03/scania_touts_et.html

 

[AlkyUser]

Thanks for the input,
It is looking at just leaving at 12 to 1.

The cam is 114.5 lobe seperation, though it is slightly reduced due to the 1.6 rockers I am running.

I am not changing the cam unless I run into problems.

I agree with the cranking pressure of will be around 250psi.

In theroy at half manifold vacum I will be running 8 to 1 compresion, using `12 to 1 static compresion.

I have ran 50 degrees timing (at a fuel consumbtion increase) and 17 to 1 air fuel ratio on 87 octane with no sign of detonation at 40kpa. (9.4 to 1 static)Cruise speeds.

I have never seen detonation in a motor that was not under low vacum.

A friend of mine moves his race car around the yard that is 13 plus compresion ratio on 87 octane, now that he is not racing. No problem, the motor never sees load.

As long as the injection system works and delivers it fuel corectly it should work. I purchased the most expensive injection system (FJO) This system allows for laptop maping of rpm, map, tps and maf.

I will post the cranking pressure and more info after the holiday season when I get time to. It will be spring before I will be able to post any faliure or sucsess stories of actual high way driving.

 

[globi5]

It's probably wise to stay on the safe side.

However, I just would like to add that it is temperature and not pressure that leads to self-ignition (detonation).
And, I just did a quick calculation and if you compress air adiabatically with a compression ratio of 14 the heat of evaporation of octane (gasoline) lowers the temperature by 5% (A/F=14.7). But if you add methanol instead, the heat of evaporation of methanol lowers the temperature by 45% (A/F=6).
So, I doubt that you run into detonation problems with pure methanol at a compression ratio of 14, given the substantial cooling effect and high octane number of methanol. Besides with methanol you can still add water (and further increase cooling effect), which is not really an option with gasoline.

 

[AlkyUser]

I totally agree.

Pressure and heat go hand in hand.

Increased compresion pressure causes some of the heat.

Heat from increased compresion pressure also causes more compresion pressure.

I was wondering if you could do the following calculation for me as I am not a engineer. I don't fully understand heat of vapourazation and latent heat.

What will the intake charge to the cylinders temperture be if?


80 degree F air coming into the throttle body -

And there was 130 lbs per hour of gasoline -

And 45 lbs per hour of 100% Methanol -

This is my aprox 6000 rpm fueling requirment. I believe there to be about 550 cfm of air flow at this engine speed.


Adding water is something that would be a patch if I had to much compression.

From running a small engine with home made ethanol with 20% water, I could safely say there is more power in a lower compresion straight fuel engine opposed to a water down mixture running in a higher compresion ratio engine.

Normally asperated speaking only, supercharging adds ve and mostly likely more boost with water would be a diferent matter.

 

[globi5]

In my previous calculations I didn't calculate the reduction of the inlet temperature. I assumed, that none of the fuel evaporates before the inlet valve closes (which admitably is not quite the case.) However, I believe it was a fair calculation to show the cooling effect of methanol in comparison to gasoline. In addition, I also did not take into account the heat capacity of gas or methanol, once it is evaporated, which would also reduce temperature nor did I take into account that some of the air will cool down due to the cool cylinder walls.
(Since the A/F ratio is about 2.4 times higher and the heat of evaporation is about 3.6 times higher, pure methanol cools the air almost 9 times more than pure octane would (I only have the number for octane).)

Btw, the rise of the temperature based on compression ratio inside the cylinders is relatively easy to calculate:
T_TDC = T_inlet * CR^0.4 (adiabatic compression of air). (Assuming a VE of 100% and no evaporation of fuel inside the combustion chamber).

Especially if you use methanol it won't all evaporate before reaching the cylinders but I can show you how to calculate your example. (I'm using metric units).

130 lbs/h = 0.982 kg/min
45 lbs/h = 0.340 kg/min
550 CFM = 15.6 m^3/min = 18.7 kg/min (air density of 1.2kg/m^3)
80F = 300K (which is the same temperature I used in the previous calculation).

You can assume a tank filled with a mass of air, a mass of liquid gasoline and a mass of liquid methanol.

Heat content of air is:
Q_air=m_air*c_air*T=18.7kg*1.005kJ/kg/K*300K=5638kJ

Heat required to evaporate octane is:
Q_gas=m_gas*h=0.982kg*300kJ/kg=295kJ

Heat required to evoporate methanol is:
Q_meth=m_meth*h=0.340kg*1100kJ/kg=375kJ

T=(Q_air-Q_gas-Q_meth)/(m_air*c_air)=264K

So if it would indeed all evaporate and all the fuel would take heat of the air and not from the manifold (which is obviously not the case) the temperature would be: 16F.
(However, this result also shows that a significant part of the fuel hasn't evaporated when entering the cylinders).

 

[AlkyUser]

I assembed the motor it has 12.4 to 12.7 to one compresion. and has a cranking pressure of 175 to 185psi. I ended up taking the heads in and surface grinding .026" off -> dart says U can flat mill .060" - .030" I would of been into the intake seat also they were advertised at 49 cc - I got 53cc

When the compresion ratio is high like this (small chambers) a few thou. in deck hieght or a cc or 2 will throw off your #s.

The pressure seems low it may have to do with unbroke in cylinders, do have some audiable leak by into the pan ( i have no leak down gauge just going with compressed air into the spark plug hole)

May also be the lower cranking speed but I was predecting 250psi.

 

[AlkyUser]

I would like to update this post, as I hate threads that have no ending. I am going to add my findings so far.

I got some more acurate measuring devices to cc the heads. You would think veternarian syringes would be close to each other but they are not.

Final cc of the 49 cc heads after surf grind is 46 - 47.
Assmbled engine static c.r. is 12.85 - 13.25 to 1

Cranking presure went up to 280psi after a short run in.

Have not ran the engine with alchol yet, but have ran some 94 octane with Torco Acelerator (Racing fuel Concentrate). Mixed at a ratio to give a 105 Octane ron mon av. fuel.

MWPC was correct on knock on warm restart. Even with the higher octane. Got rid of alot of it by delaying the fuel injection by about 3 turns of the engine, this purges the hot air from the intake and the cylinders and also alows for about 4 in of vacum. Also ramped the fuel pulse width so that It did not get a intial dump of cranking fuel.
Still ping at 0 degree timing, will be adding a push button and starting on a sniff of meth/water then the cranking gasoline.

Pardon my terms but if you add squezze you also add suck.

My ve table needs to be over 100% in the 2500 - 3500 rpm area. Peak torque with a tpi engine and factory cam.

It pulls very hard but is not a screamer.

As far as eficency, all I can say at idle under the same conditions (cam displacement) it pulls 20 inches of vacum opposed to the 17 before. At cruise 15kpa more vacum.

I expected 15% more eficency out of the engine and it looks promising. Will be doing hp and fuel consumption runs in about a month.

I am not going to get by on straight meth for enrichment.
From what I gathered 1/3 meth plus pump gas = efective 104octane.

Since the high ve in the mid range I need the equilvent of of 116 octane. Yes it pings in that area on 105, but as rpm goes up the engine is not Knock limited up at the higher engine speeds.

All testing was done in plus 5 degree cel. weather so I am sure I am going to be adding water to the meth.

Something I found with timing, in the area of knock with this engine, it will still ping with as little as 5 degrees of timing, pulling out a few degrees will not make it go away. Timing retard will not bandaid excesive compresion presures.

I am trying to be carful as I have the knock sensor set to pull out 2 deg. per ms up to a max of 30 degrees but still hear ping.

You know you have a engine very closely match when the ping at 2500rpm sounds like a ringer, not a radom ping here and there like a factory engine.

 

[SMOKEY44211]

It may be a little late in the game for this but have you considered reverse flow cooling? I have done a # of sb chev using this modification with good results. 11:1 cr with iron heads on 93 octane.--------Phil