Quote:
Originally Posted by E4ODnut
|
That is a good collection of information. I only noticed a couple of errors.
Like the BSFC maps, the emissions chart doesn't show the test load level or the relative spark advance
When you start the spark is more important in matching combustion speed with the piston speed than is the AFR, and the advance also has a significant effect on NOx in particular because of how much it effects combustion temperature.
A higher combustion temperature and pressure (up to the point of the detonation limit) is much more efficient than a lower combustion temperature, and in that way results in reduced BSFC and better fuel efficiency.
The mechanism by which this works is to speed up the burning of the air/fuel charge within a more mechanically advantageous range of the piston movement after TDC of the compression stroke.
The added heat energy during compression is what increases the burn speed.
Starting the spark earlier (before TDC compression) results in more heat and expanding gases created before peak compression.
The piston goes on to compress this hotter and more expanded charge that in turn adds significantly to the heat of the burning charge. Compression of charge adds heat energy. The dynamic compression goes up with increased spark advance. The temperature in the chamber goes up, even though EGT can often drop a bit due to less late burning in the exhaust port.
This higher temperature in the chamber exerts more force on the piston in the critical crank-to-rod angle that most efficiently transfers energy to the crankshaft. Making the speed of this expanding gas closely match the speed of the downward moving piston is the holy grail of engine tuning, for both power and economy.
Now that this subject is covered I feel the need to bring up something I have observed on various online forums, including this one.
Over and over I see far too many extremely knowledgeable and academically qualified individuals (perhaps armchair physicists) jump authoritatively into discussions concerning engine efficiency and the amount of fuel that is being burned during a given set of conditions in a running engine. These instances pop up often in discussions concerning the ability of HHO to effect a change in engine efficiency or fuel mileage, where the supposed "debunker" points to research data showing that less than 1% of fuel remains unburned as tested before the catalytic converter. The assumption being that there is not enough extra fuel to be better utilized during combustion in order to account for an increase in output or efficiency. The HHO proponents state their claim (incorrectly) that the HHO burns more of the fuel, and therefore increases fuel efficiency.
The % of fuel burned before the catalytic converter isn't the issue and never was.
What counts is WHEN the fuel is burned.
A tuner can change the output (BSFC) of the engine drastically by simply changing the spark advance and keeping all other variables the same.
If you take the "debunker's" claims as absolute reasoning, then every engine tuner out there has somehow broken the law of conservation of energy.
There are several ways to manipulate the burn-rate in the chamber, ranging from static/dynamic compression changes, fuel additives, cylinder head modifications, heat rejection rate, AFR, fuel additives, and more.
None of these violate any law of physics. The disconnect is that so many very knowledgeable people have an incomplete understanding of how the engine is actually working and even less understanding when it comes to manipulating it to effect meaningful changes beyond theory.
In power tuning N2O is used as an oxidizer to both allow extra fuel to be burned, and an increase in air/fuel burn-rate, both in order to increase power output. N2O is only 36% Oxygen by weight and in practice effects a drastic change in burn rate that requires an adjustment to the spark advance in order to stay within the physical limits of the system.
I'm pretty sure that HHO is around 88% Oxygen by weight, making it a very formidable oxidizer, and theoretically very little HHO mass would be required to effect a change in burn rate at a low engine RPM and VE.
To head off any incorrect speculation on this issue I'm going to state that yes you can add small amounts of N2O and drastically effect burn-rate without the addition of any extra fuel, so no, the use of an oxidizer does not necessitate extra fuel use, though many people have had problems with the factory adaptive fueling routine adding the extra fuel automatically.
I have yet to see this addressed correctly by either side of the discussion. I only used this example of the HHO discussion to point out where a lot of efficiency discussion gets fundamentally derailed by an apparent lack of understanding of the power transfer process as it happens in the combustion chamber.
This is something that fundamentally undermines the usefulness of many BSFC maps out there, and especially those that lack certain specifics about the operating parameters of the engine during the test.
BSFC can be changed on the fly by changing the engine management parameters, even by your driving style.
Any particular load cell on a BSFC map could represent a vehicle holding a steady speed, OR a vehicle accelerating depending on road conditions at the time. This shows that under identical calculated load conditions the piston speed can vary, and this the optimum tune will vary along with that.
Again, before incorrect speculation happens, I'm going to say that RPM is only representative of a given piston speed when measured at a steady state. You are rarely at a true steady state while driving, and thus there are significant changes (accelerations) in piston speed per each combustion event as engine load and power output constantly search for equilibrium.
For any given combustion event a falling average RPM of 2000 RPM will have a much slower piston speed than a rising average RPM of 2000 RPM.
This is another one of those obscure facts that a tuner must know to be successful at matching burn-rate to piston speed.