Fuel injection increases volumetric efficiency?

[Circlotron]

Maybe a subject for a separate discussion, but I wonder if VE would be affected if you replaced a port injector with a GDI type and bumped up the fuel pressure to what they normally run. Presumably the fuel droplet velocity would be way higher.

[LoganD]

Maybe a subject for a separate discussion, but I wonder if VE would be affected if you replaced a port injector with a GDI type and bumped up the fuel pressure to what they normally run. Presumably the fuel droplet velocity would be way higher.

Unfortunately "bumping up" to 250 or 300 bar fuel pressure isn't a simple thing. GDI fuel pumps require a lot of engineering because they are mechanically driven and require a very precise pump pressure profile (generally driven off of a cam lobe) to give consistent spray patterns.

Air/Fuel ratio is by mass, so if you're at 13:1 AFR then the fuel is only 7.14% of the Air/Fuel mixture mass. Speeding 7.14% of the mass up significantly probably isn't going to help much when the air is already moving at a good clip.

[Truckedup]

So, if emissions isn't a factor, and top end power is all that matters, carbs and injection should be close? Yes?

[Ken_Parkman]

According to the Heywood text the net effect of fuel vaporization is an increase in VE "by a few percent". Pressure atomizing injectors are not a good system for vaporizing so the effect is less than a carb which supplies an emulsified mixture. That is a reason why carbs are so hard to beat at a given design point, all other things being equal. Also why logically the more you do to vaporize the fuel in the inlet tract, ie injectors further up the tract and higher atomizing pressure, can make more power.

[MadBill]

I would expect the net effect of vaporization would depend on the intake air temperature and the volatility and Specific Heat of the chosen fuel. 410" alky Sprint cars use 'down nozzle' injectors right at the intake seat, which would suggest that the charge displacement effect of the what, ~ 5:1 AFR outweighs the potential charge density increase due to evaporative cooling.

[Circlotron]

Also why logically the more you do to vaporize the fuel in the inlet tract, ie injectors further up the tract and higher atomizing pressure, can make more power.

Would heating the fuel in the fuel rail be a good thing here?

[Ken_Parkman]

Yep, Heywood mentions that about gaseous fuels and Methanol vapor; the VE goes down significantly. I don't know about heating the fuel in the rail, don't know enough about the vaporization process.

[groberts101]

Also why logically the more you do to vaporize the fuel in the inlet tract, ie injectors further up the tract and higher atomizing pressure, can make more power.

Would heating the fuel in the fuel rail be a good thing here?

I would agree with the generality of what Ken has said in the above quote but in actuality this has been debunked many times over and sometimes power can be lost due to the fact many engines prefer a mix of varying fuel droplet sizes. Plus a valve seat and chamber design which either helps or hurts mixture homeginiety and pressure recovery can easily ruin what was once a well executed plan near the inductions point of fuel entry.

As to preheating the fuel.. sure similar to a heated plenum giving the combustion space a head start on the vaporization process. A non-bypassed e-pump has known fuel heating issues like that too which has been known to worsen vapor lock in carb application and reduce an engine designs overall octane tolerance. Unfortunately like most all things ICE related that preheating comes at a price when pushing the tune(increasing cylinder pressure) or loading the motor in a manner which reduces rpm gain to the point of combustion space heat saturation. Which then quickly erodes away at the engine designs octane tolerance.

Personally speaking I'd prefer colder fuel temps to be able to more effectively push boundaries with compression and added spark lead which generally improves average power production. Have built several cars with fully insulated fuel systems and cool cans and they give greater tuning latitude before things go south from an octane requirement standpoint.

[MadBill]

Yep, Heywood mentions that about gaseous fuels and Methanol vapor; the VE goes down significantly. I don't know about heating the fuel in the rail, don't know enough about the vaporization process.

Think Honda did that with their F1 engines 20-some years ago. IDR why..

[groberts101]

Yep, Heywood mentions that about gaseous fuels and Methanol vapor; the VE goes down significantly. I don't know about heating the fuel in the rail, don't know enough about the vaporization process.

Think Honda did that with their F1 engines 20-some years ago. IDR why..

IIRC, some of this has to do with the burn time available. A 20,000 rpm engine has very little time to break down and consume larger droplets compared to a 7,000 rpm engine.

[gruntguru]

Fuel emerging from a carb jet into the moving air stream, will do at least two things:
it will evaporate and thereby chill the inlet air density (depending on how much fuel has actually evaporated).
and
The fuel vapours will displace a certain volume of inlet air, reducing its volume.

So now we have a trade-off; a reduced volume of air/fuel mixture but with a greater density.
Have we increased or reduced the mass of the air/fuel mixture?

The added fuel vapour has increased mixture mass but has reduced inlet air mass.

Port injection will deliver a finer droplet size and will not displace air volume, but we will not
benefit from evaporative cooling. Direct injection would be even more efficient.

Did some back-of-napkin calculations. Typical fuel has a latent heat of vaporisation around 300 kJ/kg. At 12:1 AFR, if you can completely evaporate the fuel this will provide enough cooling to cool the air by 25*C, increasing its density by about 10%. The evaporated fuel occupies 1/55th of the total volume ie reducing the "air" per unit volume by less than 2%. So the net result of evaporating the fuel is an improvement of about 8%. (Fuel that does not evaporate has a negligible volume compared to the charge air - about 1 part in 10,000)

This will be significantly better for Ethanol (840kJ/kg) and Methanol (1100 kJ/kg).

Water (2250 kJ/kg) has a massive cooling effect - if you can get it to evaporate!

[gruntguru]

Yep, Heywood mentions that about gaseous fuels and Methanol vapor; the VE goes down significantly. I don't know about heating the fuel in the rail, don't know enough about the vaporization process.

Think Honda did that with their F1 engines 20-some years ago. IDR why..

That was done to improve thermal efficiency under the fuel limited formula applied during the last years of the previous turbo era (1988). Most probably, the hot fuel (80*C) was burning faster and more completely. Honda also increased charge temperature (to 70*C) for the same reason.

https://www.scribd.com/document/1237322 ... 68E-Engine

[cjperformance]

What about the effect on the velocity of a given 'parcel' of air when you cool it with vaporizing fuel.
You displace some air volume due to the fuel injected, but cooling the air due to vapourization increases density which should give a reduced velocity, but does the gaseous and residual volume of liquid fuel make up for the reduction in air volume taken up and negate(or nearly) any effect of this.?

[David Redszus]

That was done to improve thermal efficiency under the fuel limited formula applied during the last years of the previous turbo era (1988). Most probably, the hot fuel (80*C) was burning faster and more completely. Honda also increased charge temperature (to 70*C) for the same reason.

With the fuel limited formula, then in use, Honda went to higher density fuels to improve fuel mileage and possibly avoid a pit stop. But the aromatic fuel blends were difficult to evaporate at higher rpms (and reduced time) so the fuels were heated to assist evaporation.

Even heated fuel will cool the inlet charge due to evaporation and have virtually no effect on flame speed.
Increased inlet air temperature will have a significant impact on flame speed.

The factors that affect fuel evaporative cooling are:
Fuel Heat of Evaporation
Stoichiometry
Fuel Specific gravity
Fuel Specific Heat
Fuel vapor density
Fuel temperature
Inlet air temperature
Fuel enrichment factor
Fuel fraction evaporated

All which will result in changes in each of the following
Air mass
Air density
Air volume
Fuel mass
Fuel volume
Vapor volume
Total volume
Inlet charge expansion

[gruntguru]

What about the effect on the velocity of a given 'parcel' of air when you cool it with vaporizing fuel.
You displace some air volume due to the fuel injected, but cooling the air due to vapourization increases density which should give a reduced velocity, but does the gaseous and residual volume of liquid fuel make up for the reduction in air volume taken up and negate(or nearly) any effect of this.?

The liquid fuel displaces 1/10,000 the of the charge volume 0.01% so no effect there. As droplets evaporate they cool the surrounding air and shrink it by an amount which is greater than the extra volume occupied by the fuel vapour. So the overall effect is roughly a 6% increase in oxygen density, a 6% shrinkage of charge volume and a 6% reduction in velocity (assuming 100% evaporation).

Note. Can't edit my earlier post to correct a calculation error. Air density increase should be 8% and fuel vapour volume is 2% giving 6%oxygen density increase.