WetLeather Motorcycle Rider Information

Martin:
The cold mix vaporizes less and is harder to ignite, so it doesn't burn cleanly. That means less energy (all the way down to doesn't burn at all) is available. At least enough extra fuel must be added to get enough to burn that the engine generates more energy than it consumes in friction. If it is intended that the engine be safely operable, enough extra fuel must be added across the range that the engine achieves reasonable power output.

Michael:
The reason fuel enrichment or choke devices work to get the engine running is the simple 'throw more fuel on the fire' principle. Enough gas gets dumped into the combustion chamber that it simply must by near accident create enough air fuel vapor mixture to ignite. Or flood. One of the reasons the cold start emissions test is so difficult to pass is that all that raw unburned gas gets pumped through the engine and out the exhaust. It's a cludge and a horribly inefficient way to make a motor run. After you get it to start the combustion in the engine warms the intake tract walls and the droplets of fuel no longer condense on the walls of the intake enroute to and past the intake valve thingamajiggy. Looking at the Air Fuel Ratio when performing a cold start on a carburetor equipped engine at or immediately following engine startup you might see an AFR of 7:1 or maybe 10:1. That's VERY rich and shows up as horrendous unburned hydrocarbons out the tailpipe(s).

Remember when Auntie Em used to start her ol' Chrylser up in the garage and leave a huge black soot mark on the door before she backed up and destroyed the door? Rich, rich and richer. The ol' 440 ran but it ran like crap. Until it got warm.

Martin:
At higher compression ratios the mixture burns better, so less extra fuel need be added at higher throttle openings. Thus the (so far experimental) variable compression diesels, the effectiveness of simple enrichening circuits, and the need for larger idle airflow in fuel injected systems.

Michael:
Compression ratio has nothing to do with how the mixture burns. Or, well, ummm...It only affects the specific power output of that specific fuel in that specific engine. High compression makes high horsepower by allowing you to slow the burn and make the power stroke last fractionally longer. You squish the same volume of fuel/air into a smaller squish zone, fire the plug and the flame front travels faster across the top of the cylinder making it go downwards faster and et violins you make more horsies. Slow the burn down and you can jam more fuel in on the suck stroke so that on the squish stroke you squish more fuel and bang it slower so it goes faster. Or something like that. It's really complicated and I'm getting a headache now so I'll move on. Know why regular gas pings in that ol' Chrysler? Cause it burns too fast. Premium burns slower, no ping, more power _in a high compression motor_. If you have a motor that the manufacturer says to run regular in, do so. It'll make more power and give you better fuel economy to boot.

Now where was I? Oh yeah, intake and exhaust valve opening and closing timing and rates and flow pattern in the intake, burn pattern and flame front propagation during the burn in the cylinder and exhaust backpressure or lack of same can and do influence how effectively the mixture burns. It's all weird science that requires a fine balancing act on the part of the engine engineer who engineers the engine. (giggle)

Fuel injection changes everything. For starters most automotive electronic fuel injection systems don't perform direct injection into the cylinder. But, they do inject the fuel in a very precise way as close to the back of the intake valve as possible. Computer control of the injector means that rarely do the manufacturers need to provide a secondary method of fuel enrichment. (Old VW Rabbits and their ilk with Bosch CIS don't count.) The ECM determines the engine temp using the coolant temp sensor (cold) engine RPM (cranking) and using the tables provided by those wunderkind back at the factory says "oh, Martin is trying to go to work, I think I'll let him and set the injector pulse to allow this stone cold engine to fire _NOW_. And it does. Very rapidly the same ECM will lean the mixture as much as possible to allow for smooth running and simultaneous low emissions. Rocket science!

'the need for larger idle air flow in fuel injected systems'?
All of my experience with GM, Chrysler, Ford and Bosch injection systems show me that the throttle plate is set at near zero opening and a separate idle air circuit is provided to fine tune the idle speed depending on load (AC on or off or other electrical loads like rear window defroggers) and engine temp. The ECM makes injector pulse decisions to maintain the fuel mixture at the leanest safe point possible. Using a wide-band O2 sensor and a Tektronixs DSO, I've seen air fuel ratios approaching 19:1 at idle on small block Fords. Tip the throttle and the AF Ratio jumps to 13:1 or even lower. It's really important to note that AF Ratio and Compression Ratio are two distinctly different things and have separate and dramatically different effects on performance.

Ryan:
Wouldn't higher compression ratio cause a little better atomization due to the heating of the intake charge? A 13:1 engine at TDC will be a lot hotter than a 8:1 at TDC....if you see what I mean.

Ed:
Higher compression raises the ultimate combustion pressure and temperature, making more BEMP (Brake Effective Mean Pressure) to push the piston down, and thus increases the "thermal efficiency" (the amount of work the power stroke can do) of the combustion cycle. It also increases the rate of burn (chemical reactions happen faster at higher temperature) requiring higher octane fuel to slow the burn rate down (more uniform pressure throughout the power stroke) and prevent compression ignition.

The mixture has to be enrichened when the engine is cold because 1) the incoming cold air charge is more dense requiring more fuel to achieve the correct fuel-air ratio 2) the cold fuel requires heat to vaporize; less heat is available when cold, therefore less of it evaporates to a spark-ignitable gas, therefore the mixture is inherently leaner and needs to be enrichened with extra fuel.

Fuel injection also increases the degree of fuel atomization and increases the percentage of the fuel that vaporizes prior to ignition, thus effectively making the mixture in the cold cylinder richer, while ACTUALLY making it leaner in terms of the total amount of fuel needed to be injected when cold; good for emissions and cold start performance.

Michael:
Much more highly qualified brains than my own have done a ton of work on the whole issue of suck/squish/bang/blow. Here's what I understand the dynamics to be:

Atomization occurs for the most part at the carbietooter (after all, isn't a carb a simple atomization device?) with some additional mixing of the air/fuel mix happening between the carbietooter and the back of the intake valvethingamagiggy in the swirl of air/fuel as it gets sucked into the motor. Or in the case of an injected motor it occurs at the point of injection. The intake charge is heated not in the combustion chamber but in the intake tract (for all intents and purposes). If you heat the charge in the combustion chamber it goes 'boom' before it's 'sposed to. In fact, using the air/fuel mix to actually _cool_ the combustion chamber is common practice, especially amongst turbo motor geeks.

If you want to talk about the effect of combustion chamber shape on flame propagation rate, you're talking about something way different from compression ratio. Swirl within the combustion chamber as the shape and volume changes and that maintains the intake charge in a near 100% atomized condition until the moment that the flame front is ignited, is something that designers of combustion chambers strive to attain. Note the word 'strive'. Very few engine designs really manage that feat. F1 motors perhaps, some other really high dollar and high performance motors maybe. Mom and Dad's Oldsmobubble? Not likely. The Quad Four motor did manage to do a decent job of it though.

I haven't heard of anyone trying to change a swirl pattern with a bump in the compression ratio. Swirl is changed with valve angle, intake tract angle, piston and combustion chamber shape (cylinder head work...works) and exhaust valve angle and exhaust extraction efficiency. Oh, and valve timing, lift and duration. Increasing the compression ratio would be a result of increased swirl efficiency causing better control of the flame front and allowing the CR to climb.

Hotter (as you state above) simply means you need to use a slower burning fuel to avoid the dreaded pre-ignition syndrome or retard the spark timing to do the same. You can make _any_ engine experience a too fast (in relationship to the crank angle) burn if you light it off early. Advance the ignition enough and a 5:1 running on white gas motor will tinkle like a mad dog wearing a cow bell.

What we do know about pre-ignition is that it is an uncontrolled ignition event. Bad ju-ju. When the engine designer locates the spark plug in the cylinder he/she is stating "start the flame _here_". When pre-ignition occurs the hot little air/fuel mixture says 'hey! I think I'll light off _now!_ and to heck with when you want me to go boom." The flame front doesn't travel in the correct pattern, hot spots occur on the tops of the pistons and expensive holes happen therein. Ignition is a waiting game. You have to wait to fire the plug until exactly the right moment so that the flame burns at maximum efficiency but not wait too long or you'll have suckage in the power department.

I've often wondered why older 4stroke moto engine technology didn't include the same kind of vacuum advance or weight based mechanical ignition advance mechanisms as we saw on the same generation of automotive engines. Or did they? I don't know for sure, I was riding 2smokes until 1995. Now a 2 stroke motor has some really weird sh*t happening in the intake/combustion chamber/exhaust departments. But, like diesels, let us not go that way.