Fuel, Octane, and Ignition Timing

[C5Performance]

Fuel, Octane, and Ignition Timing

I would like to discuss fuel and timing because Octane, fuel burn rate, ignition timing, and load are very much related to one another.

We'll start with the basics. Jump in anytime with questions or comments. When we understand how these items relate, it improves our ability to get our vehicles running their best.

Let's pretend we started our own fuel company. How do we decide what chemicals to use and what Octane reading is appropriate? Will our fuel be compatible with our customer's vehicle?

Our customer expects easy starting, smooth idle, and lots of power.

So first we must discuss PCP. That's Peak Combustion Pressure. The term describes at what crankshaft angle we get peak “pressure” pushing down on the piston. This angle is 16 degrees after top dead center (ATDC).

If the pressure peaks too soon or too late you lose the mechanical leverage needed for great power.
All fuel burns at the same rate all the time, so we just mix up some some stuff that works with 10 degrees of timing advance at idle, and 40 degrees at full throttle and our problem is solved, right?

Not so fast!

Fuel does NOT burn at the same rate all the time!

We must consider our customers needs:

engine compression==> Do they have 8:1 or 12:1?
vehicle load==> Is their vehicle light or heavy?
gearing==>Is it geared for mileage or power?
terrain==>Do they ride on flat ground or tall hills?

**BREAK TIME**

Next we'll move to Octane...something we've all read about and maybe don't totally understand.
If you have questions please ask now...

 

[joedrum]

hmmmm so cam timing of valves plays huge roll here ? ...

and changes attitude of motor if PCP is somewhat concrete 16 degrees ATDC

 

[C5Performance]

joedrum » Fri Sep 20, 2013 11:13 pm[/url]":1grdh5ix]
hmmmm so cam timing of valves plays huge roll here ? ...

and changes attitude of motor if PCP is somewhat concrete 16 degrees ATDC

I have read that 15-17 degrees ATDC is the magic number. Gary at PowerArc said any expansion of fuel after 40 degrees ATDC is completely wasted, since there is no mechanical leverage anymore.

Think of you pushing your grand daughter around a Merry Go Round. If you grab the railing too soon you actually slow her down, if you push too late you almost fall over and didnt really make her go faster.

BUT...there is a magical point that you give that thing a shove and she really takes off....that is exactly the same principle we are talking about here.

Since we are talking about the igniting and burn of fuel at compression stroke (both valves are closed), in my mind cam valve timing will not affect this.
It's all about matching PCP so it occurs when the piston & crank are in a position to USE the pressure to make more torque.

After that point, you begin to quickly lose mechanical leverage and power starts dropping quickly. By 40 degrees ATDC you have ZERO leverage left (so you are wasting gasoline and not making more power).

Did I get that correct?

 

[joedrum]

yes I see ..but it seems this effect is somewhat in play with drawing fuel and exhausting too .. as in you cant compess something that not there or compress something that is there without enough mechanical leverage to do so....im sure my questions are a bit off the mark ... but with all the cam switching going on its nice to throw this in if it fits in the discussion some here ...there are several here me included trying to sort things here ... and this discussion may help

 

[dan filipi]

Good discussion.
The merry go round example makes it really easy to understand.

So now how does fuel burn rate and octane come into play here?

 

[Dave_J]

Cam timing relates to static and dynamic compression ratio. When you assume that with both valves closed and sealed, the volume of the cylinder's stroke and bore, pistons crown/dish volume, heads chamber volume and headgasket volumes all plugged into the computer this gives you the static compression. Things like ring leakage and cam timing is what gives you the dynamic compression ratio.

A 91 Jeep 4.0 liter has a Static of about 9.2:1 but because the (CRAPPY) cam timing has so much overlap and duration the dynamic ratio is like 6.8:1 It runs on regular gas. Now put a 4.2L (258 ci) Rambler/AMC/JEEP crankshaft in and give it an overbore of 0.060 and now you have a 4.7 L. Put a CompCams 268 degree cam in and now you must run super as the Dynamic ratio is around 9:1, BUT the Static is up to 10.25:1

To build an engine to run best on E-85 the Dynamic compression ratio should be up around 11.5:1 or MUCH higher if piston oilers and other stuff are employed. My 2008 Burgman 650 runs best on regular pump gas and it has 11.2:1 compression. That 11.2:1 is most likly the static ratio, I am not sure.

This is only a quick off the top of my head answer about compression ratio.

 

[NewWinger]

1. I understand this applies to spark ignited engines.
2. How would one apply these maxims to diesel with intercooling and turbo charging that negates pumping losses of a restricted intake like typical si engines?

 

[slabghost]

Diesel is entirely different as ignition occurs by compression. No spark needed.

 

[NewWinger]

Uhm? Well yeah? Slag, but timing is critical with diesel also.
In fact many diesels now days have multi pulse or injection shaping which require accurate timing as well. I was just asking how this relates to things like turbo charging where your pumping losses are almost non-existent compared to naturally aspirated engines.

 

[slabghost]

Due to the very different characteristics of diesel fuel compared to gasoline. There really isn't much comparison. The injectors of a diesel are the distributor and source of timing. Injection of gasoline differs because the stroke is shorter and it requires a spark to ignite.

 

[C5Performance]

dan filipi » Sat Sep 21, 2013 3:52 pm[/url]":1tgwdhzj]
Good discussion.
The merry go round example makes it really easy to understand.

So now how does fuel burn rate and octane come into play here?

Dan, I am jumping back to your question so we don't lose 90% of our readers.

When discussing fuel and ignitions, let's keep it simple and pretend we have just two options:

1)We are trying to design fuel that burns at a rate which is compatible with a GL1100. In my example, we need to manufacture gasoline that can be ignited at 10 degrees before TDC (at idle) and burn at a rate that gets us maximum PCP (pressure) at exactly 16 degrees after TDC.

2)We could manufacture fuel that's cheap, and does not have a burn rate that makes peak power. Instead, we could design an ignition that could slow down or speed up the timing to match the fuel to avoid engine damage.

If you don't understand this, read it again. We can make fuel to match our customers ignition, or we can change their ignition to match the fuel.

If everyone understands this, I will move forward. If not, PLEASE jump in and ask a question.

 

[C5Performance]

Dan mentioned Octane. Many people including me thought octane was some kind of power booster.
I often wondered why we didn't just dump a bunch into our fuel to begin with.
Well high octane fuel can actually contain less energy than low octane fuel.

Buts lets discuss detonation first. If fuel burns too fast, it can cause knocking. We also are trying to design a fuel (remember or pretend company) that reaches PCP (peak cylinder pressure) at exactly 16 degrees ATDC. So how would we use octane to achieve this goal?

We mixed up our first batch of gasoline and it sold very well. Most people liked the way their GL1100 ran with it. We made it affordable, with good "cold starting" qualities. So why mess with it?

Well our sales dept got a call from a few hot rodders and they have detonation when using our fuel. They have high compression engines, run under high load conditions, and love to run 44% timing advance. Our fuel is burning too fast for their engine!

Now we can talk about Octane. We are going to try slowing the burn rate of our fuel and decrease the chances of flash combustion. I am not going to explain the different chemicals used to obtain octane but lets agree that adding more will allow our "racer" buddies to burn our gasoline without engine damage. We can mix our new "Super CW" gasoline with higher octane levels and slow down the rate of fuel burn. For engines with higher compression, higher load, higher temperature, and more aggressive timing we now have a fuel they can use.

It all comes back to this one statement: For optimum performance and efficiency we must match fuel burn with ignition timing.

So what if we can't use more Octane? What if the government limits what we can add to our fuel? How will we stay in business??

The next discussion will involve adjusting our timing to match the fuel we have available!