Volumetric Efficiency

[digger]

I don't think a lot of fuel gets scavenged into the exhaust as that would cause serious explosions in the exhaust. Never had that happening.

Air when rushing into a cylinder has the tendency to keep on going along the set out path from its intake port. And this happens not to be in the direction of the exhaust.

Depends on valve and port layout.

[David Redszus]

Agree for most sequential systems. Anything much over 25% duty cycle will see fuel getting scavenged. For those that want to avoid this, it is entirely feasible to increase injector size to keep DC <25%. For boosted engines it may be necessary to run 2 staged injectors per cylinder to get adequate light load accuracy.

Injection tuning is much more involved than merely selecting the injector size and duty cycle.

Let's take an engine running at 5000 rpm, with a End of Injection (EOI) point at 70 CS deg,
Injector Timing of 9.0 ms, which yields a Duty Cycle of 37.5% and an Injection Duration
period of 270 deg. None of the injected fuel will see the overlap window and no fuel will be
lost out the exhaust.

Now let's run the same set up at 7500 rpm. The Duty Cycle becomes 56.3%, and the injection
duration period becomes 405 deg. Now the injection period straddles the overlap window
and a substantial amount of fuel is lost out the exhaust.

At 10000 rpm, the Duty Cycle becomes 75%, and the Injection Duration period is
now at 540 deg. Again, fuel is lost out the exhaust during overlap.

Increasing the Injector Timing makes matters worse; we need to reduce the timing and
Duty Cycle but will run much leaner if we do. We need larger capacity injectors and/or
additional injectors so that the duty cycle can be reduced to prevent fuel loss.

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

We can alter the fuel map to provide a longer Injection Duration at higher rpms to compensate
for fuel loss if the Duty Cycle does not become excessive.

Have we ever seen a fuel map with a 120% duty Cycle?

[gruntguru]

Now the injection period straddles the overlap window
and a substantial amount of fuel is lost out the exhaust.

Stating the obvious here but any fuel injected against a closed intake valve is potentially scavenged during the next overlap window.

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

It sees both air and fuel although its ability to see fuel is poor.

Have we ever seen a fuel map with a 120% duty Cycle?

Only poor software coding would allow that.

[David Redszus]

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

It sees both air and fuel although its ability to see fuel is poor.[/quote]
Since the Nernst cell responds to the partial pressure of oxygen, how does the Lambda sensor respond to the presence of fuel in the exhaust?

[Belgian1979]

One could also use 2 banks of injectors of similar size. One in the port and one at the entrance of the runner, so above the throttle plate. That way, you would benefit from the longer time it takes for the fuel to travel to the valve and allow it to properly vaporize. This would prevent the need for overly large injectors with the poor injection patterns and small opening time irregularities, while at the same time allowing you to use cheaper injectors.

Basically you would only switch 'on' the top injectors above a certain rpm.

It provides the best of both worlds but with added complexity though.

[MadBill]

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

It sees both air and fuel although its ability to see fuel is poor.

Since the Nernst cell responds to the partial pressure of oxygen, how does the Lambda sensor respond to the presence of fuel in the exhaust?
[/quote]

Since it closely follows or even mixes with the tail end of the exiting hot exhaust gasses, is it certain that no over-scavenged mixture ignites and burns upstream of typical O2 sensor locations, thus at least reducing the effect? ('seeing' some of the fuel)

[gruntguru]

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

It sees both air and fuel although its ability to see fuel is poor.

Since the Nernst cell responds to the partial pressure of oxygen, how does the Lambda sensor respond to the presence of fuel in the exhaust?

On the surface of the exhaust gas electrode (which is platinum) the presence of unburned fuel species (particularly hydrogen) will react with and deplete the oxygen present.

"Operating characteristics of zirconia galvanic cells (lambda sensors) in automotive closed-loop emission control systems
Bozek, John W | Evans, Richard | Tyree, Clifford D | Zerafa, Kenneth L
SAE Special Publications , no. 910, pp. 1-17. 1992

Simple tests were performed to investigate the operating characteristics of zirconia galvanic cells (lambda sensors) in automotive closed loop 'three-way' emission control systems. Commercially available cells were exposed to typical gaseous components of exhaust gas mixtures. The voltages generated by the cells were at their maximum values when hydrogen, and, in some instance, carbon monoxide, was available for reaction with atmospheric oxygen that migrated through the cells' ceramic thimbles in ionic form. This dependence of galvanic activity on the availability of these particular reducing agents indicated that the cells were voltaic devices which operated as oxidation/reduction reaction cells, rather than simple oxygen concentration cells. Such operation explains why a cell that is used as a lambda sensor in a closed-loop control system exhibits a sixfold or greater decrease in voltage output when the exhaust gas composition changes from a slightly rich condition (lambda identical with 0.995) to a slightly lean condition (lambda identical with 1.005). It also explains why the voltage of a cell that is located downstream of a properly operating catalyst normally remains at a low level as the air/fuel ratio oscillates around the stoichiometric value but increases to a high level when ignition misfire occurs at a rate that exceeds a certain value."

[In-Tech]

Since the Lambda sensor only sees air and not fuel, it will not tell us we are losing fuel.

It sees both air and fuel although its ability to see fuel is poor.

Since the Nernst cell responds to the partial pressure of oxygen, how does the Lambda sensor respond to the presence of fuel in the exhaust?

Since it closely follows or even mixes with the tail end of the exiting hot exhaust gasses, is it certain that no over-scavenged mixture ignites and burns upstream of typical O2 sensor locations, thus at least reducing the effect? ('seeing' some of the fuel)
[/quote]

I don't want to get too far off the OP subject that I didn't want to talk about anyway. BUT(you know that means throw everything a person said away before BUT) Take a spark plug wire off(and still use a spark plug or tester so the coil fires) of anything late model and it will set a misfire code for THAT cylinder that is misfiring. How does it know? It's sequential fuel and spark and the computer knows the transport delay based on rpm and mass flow to the o2 sensor(Also why CARB doesn't allow any exhaust change that changes the distance of the o2 sensors).

But let's forget about that for a sec and talk about what happens inside the engine. The engine is still digesting the same amount of air and fuel in the "dead" cylinder and that fuel DOES burn on it's way out. So why does a misfire show lean on the o2 sensor? Because combusted air/fuel doesn't read the same as burnt fuel as I eluded to in another post. Test this for yourself, datalog both the pre-cat and post-cat o2 sensors with a spark plug wire off.

Warp, you mentioned you had changed valve overlap dramatically and tuned to the same AFR. Was the exhaust closing event the same? I would assume the exhaust system was well working before the change, could it be that it was so finely tuned that it wouldn't scavenge more? Did the extra overlap cause an "egr" situation? Anyway, not trying to stray away from the VE discussion and always like to hear what you folks have to say.

[David Redszus]

Oxygen is not the only substance that will effect an oxygen sensor. The composition
of the exhaust gas has a very real effect on both the sensor switch point and output
values. Gases such as free hydrogen will cause a lean shift in the switch point. Gases
with molecular weights heavier than diatomic oxygen (O2=32), such as most heavier
hydrocarbons (isooctane, aromatics, etc.) will cause a rich shift in the switch point of
the sensor. Some substances such as NO and SOx have very little effect while toluene
and isooctane vapors have a large effect.

In addition to excessive heat, oxygen sensors can be destroyed in several ways. They
can be poisoned with sulfur, phosphorus, silicone and of course lead. They can also
be fractured by thermal shock. Excessively rich mixtures, oil additive deposits, and
excessively low temperatures can foul the pores of the sensor ceramic tip.

Oxygen sensors can reveal a great deal of information regarding the performance of an
engine. While they cannot measure air/fuel ratio they can tell us how the engine is
running, can indicate ignition misfires, can signal overscavenging and incorrect pipes.

All race engines operate in the slightly rich range, normally using about 7.5% fuel. The
exact fuel ratio will depend on the fuel composition, the fuel preparation (evaporation),
and the chamber design which will affect completeness of burn. There is no set value
for maximum performance, but there is a typical range for race gasolines: from 7 to 8%.
Other fuels will operate at different fuel ratios.

The amount of oxygen present in the exhaust should remain constant across the rpm
range at constant load. If ignition misfires are present, the oxygen sensor will display
a ragged, irregular trace since it is reacting to the presence of unburned oxygen. It is
often possible to see fuel dripping from the exhaust pipe while the sensor indicates lean.

Camshafts with excessive overlap (at a given rpm), will overscavenge the chamber and
draw unburned air and fuel into the exhaust which will be read as a lean condition by the
oxygen sensor. Very frequently, the exhaust ports may indicate a dark and wet look
while the exhaust header indicates a tan or gray appearance. This is an indication of
combustion occurring in the exhaust header, often called pipe burn or afterburning. Fuel
that burns in the exhaust pipe may improve pipe tuning but does very little to improve
combustion in the chamber.

Similarly, a two stroke pipe (and some four stroke pipes as well) can overscavenge the
chamber and draw unburned air and fuel into the pipe which is indicated by the oxygen
sensor as a lean condition.

It is important to note that oxygen sensor output voltages will not display a stable,
smooth value. Every engine exhibits cylinder to cylinder, and cycle to cycle variations
which can be quite considerable. Consequently, there is no single sensor value but
rather a range of oxygen sensor values that must be considered.

The proper use of a racing type oxygen sensor can provide a wealth of important engine
tuning information. It can be used to balance mixture between cylinders, ensure that the
fuel delivery is stable and consistent across the entire rpm operating range and track
operating conditions, reflect changes in fuel composition and air density, and even
indicate ignition misfires. When the data is fed into a data collection logger, it forms a
permanent record of engine performance for a specific track and day.

But it can just as easily be misused.
There are no predetermined Lambda values that will produce maximum power. The best
power Lambda value must be determined for each combination of fuel, engine and state
of tune. Once the best engine power is determined, the Lambda value can be noted
and used as a control target value to ensure that performance is maintained under all
variable operating conditions.

Not all sensors are the same nor intended for the same purpose. Some are designed to
operate in lower temperature areas. Some are designed to operate in low speed air flow
locations. Some are easily destroyed by high exhaust temperatures. Some are very
sensitive to lead deposits or carbon fouling. All sensors will be inaccurate if installed in
locations that do not produce the design temperature. Remember, a heated sensor
can raise its tip temperature but it cannot cool itself.

Oxygen sensors designed for street use are rarely suitable for use in high performance
racing engines or for use with leaded racing fuels. While oxygen sensors designed
specifically for motorsports use may cost more initially, they will last much longer and
provide dependable, accurate readings.

[Firedome8]

Can overscavenge of the cylinder make exhaust pipes red hot ?

[mk e]

I don't want to get too far off the OP subject that I didn't want to talk about anyway. BUT(you know that means throw everything a person said away before BUT) Take a spark plug wire off(and still use a spark plug or tester so the coil fires) of anything late model and it will set a misfire code for THAT cylinder that is misfiring. How does it know? It's sequential fuel and spark and the computer knows the transport delay based on rpm and mass flow to the o2 sensor(Also why CARB doesn't allow any exhaust change that changes the distance of the o2 sensors).

Are you sure about that?

I have a Bosch ecu manual for the...7.3? Like 2000 vintage OEM ecus, the manual Bosch gives the OEM engineers. Anyway in that ECU movie detection is done exclusively but looking at tooth to tooth time variations on the crack trigger. Basically they look for shorter times on the power stroke and if they aren't shorter it sets a flag, 3 consecutive flags and it sets a misfire error... I think it was 3, it's been some years since I went through it.

Things may have changed but I know I tried to do exactly what you're describing when I was troubleshooting misfires in the frankenferrari but ever with only 3 cylinders on an O2 I was not able to pull cylinder data out of the O2 signal. My ecu samples at 2khz but only logs at 1khz...still, 1500 rpm that's 38 pulses/sec per sensor so 25 data points per cylinder event....I couldn't see it because the sensors I tried (NB& innovative WB) didn't react that fast.

Anyway I gave up and went to a tooth to tooth time similar the Bosch system and it seemed to work....but I dropped a valve seat mid testing so I'm sure it still needs some work.

[Belgian1979]

EGT's in the exhaust ?

[Firedome8]

I don't want to get too far off the OP subject that I didn't want to talk about anyway. BUT(you know that means throw everything a person said away before BUT) Take a spark plug wire off(and still use a spark plug or tester so the coil fires) of anything late model and it will set a misfire code for THAT cylinder that is misfiring. How does it know? It's sequential fuel and spark and the computer knows the transport delay based on rpm and mass flow to the o2 sensor(Also why CARB doesn't allow any exhaust change that changes the distance of the o2 sensors).

Are you sure about that?

I have a Bosch ecu manual for the...7.3? Like 2000 vintage OEM ecus, the manual Bosch gives the OEM engineers. Anyway in that ECU movie detection is done exclusively but looking at tooth to tooth time variations on the crack trigger. Basically they look for shorter times on the power stroke and if they aren't shorter it sets a flag, 3 consecutive flags and it sets a misfire error... I think it was 3, it's been some years since I went through it.

Things may have changed but I know I tried to do exactly what you're describing when I was troubleshooting misfires in the frankenferrari but ever with only 3 cylinders on an O2 I was not able to pull cylinder data out of the O2 signal. My ecu samples at 2khz but only logs at 1khz...still, 1500 rpm that's 38 pulses/sec per sensor so 25 data points per cylinder event....I couldn't see it because the sensors I tried (NB& innovative WB) didn't react that fast.

Anyway I gave up and went to a tooth to tooth time similar the Bosch system and it seemed to work....but I dropped a valve seat mid testing so I'm sure it still needs some work.

Do some use ion sence to detect misfire ?

[engineguyBill]

The discussion has gotten away from the original topic of volumetric efficiency (VE). VE is a theory as to how much air/fuel is being trapped in the combustion chamber, upon the intake valve closing, at high (maximum) RPM levels. The calculation has nothing to do with whether or not the engine is carbureted or fuel injected, nor the type of fuel used. A typical n/a engine may be achieving around 103 to 105% VE at high RPM, but is very much dependent upon the engine design from intake manifold to header collector and everything between.

[Stan Weiss]

The discussion has gotten away from the original topic of volumetric efficiency (VE). VE is a theory as to how much air/fuel is being trapped in the combustion chamber, upon the intake valve closing, at high (maximum) RPM levels. The calculation has nothing to do with whether or not the engine is carbureted or fuel injected, nor the type of fuel used. A typical n/a engine may be achieving around 103 to 105% VE at high RPM, but is very much dependent upon the engine design from intake manifold to header collector and everything between.

Bill,
How is that measured? Any dyno sheet that I have seen that shows VE in no way knows how much was trapped, only how much went in.

Stan