My science teacher always said "Science never sucks.... It blows" He loved that line

 

Old SStroker couldn't have said it better...basically the engine is naturally trying to "spin" and will do so freely until the point where it would probably self-destruct...

Which is why we have a throttle blade (and a rev-limiter when the blade is open), to restrict the amount of air it gets and (feed) control it. I like the analogy of taming a wild animal

 

i know of one situation where something SHOULD get sucked....or blown, yeah i guess it all depends on the point of view.

 

Same as pressure systems in the atmoshpere, High-to-Low. It wants to go towards that low pressure. Same way a wing fly's on a plane, increased speed of the air on top of the wing due to cambered shape lowers the pressure, as compared to the airstream passing under the wing being the same pressure as when the wing encountered it so it stays the same, or higher pressure than the top air. High-to-low equals wing goes upward. I'm a pilot so I can see the same effect going here.

But, isn't the engine really sucking air in, or at least its a sucking process of the pistons that starts the whole process off, therefore we can say that air is being sucked into the air by way of this process, which is Bernoulis.

I'd like to know how high the psi gets from a BBC with 1,000 hp normally aspirated at WOT, I would think it would surpass the ~14.7 psi that the atmosphere has to offer at sea level to keep pushing air into the engine. Therefore isn't there a point that an N/A engine is purely sucking air in to keep accelerating itself to a point that it maxes out even that, red line?

.

 

Haha, i thought about that right after i posted.

 

In terms of this discussion I follow all of the descirptions, and learned a few new perspectives on engine dynamics. However, I find myself wondering what exactly vaccum is. IOW, most mechanical boost gauges read "vacuum" before boost. Is this in reference to the negative pressure differences between an idling/partially open throttle blade? Thanks for any feedback!!

Tommy

 

A vacuum or boost gauge is simply a pressure gauge referenced to atmospheric pressure. 5" vacuum is simply 5" pressure lower than atmospheric. Vacuum is usually measured in inches/murcury, where boost is measured in PSI.

Re: the original question.
Piston movement in the cylinder expands the volume of the cylinder and lowers the pressure. The atmosphere wants to come in via the intake port to equalize the pressure. The throttle is the pressure regulator. The more you open the throttle, the closer the pressure in the cylinder will get to atmospheric pressure. Air at higher pressure has a greater density and contains much more oxygen. As pressure in the cylinder goes up, engine power goes up.

Fueling or the addition of fuel is simply to maintain the right Air/Fuel ratio as the amount of air/oxygen is changing with throttle opening.

This would be the case even if our atmosphere already had the correct ratio of fuel vapor in it. The power would still be throttled or controlled by the amount of pressure allowed into the engine. IMO you should think of it like this instead of the amount of fuel going in making the difference.

A diesel engine is always WOT and power is based strictly on the amount of fuel or the A/F delivered. This is completely different.

 

Not sucking, but atmosphere filling a newly created volume (cylinder on intake stroke). Idea doesn't change with RPM unless you get into harmonics, advanced valving.

And a very good N/A 1,000 HP BBC would still never make much more than a bar absolue manifold pressure (0 gauge) at any RPM.

 

The definition of the word throttle when used as a verb means to strangulate or choke (as in pressing in on the wind pipe a la the Boston Strangler). The butterfly valve or throttle plate regulates the inflow of air by limiting (or throttling) the air flow to something less than its maximum capability at wide open throttle (WOT). Whenever you open the throttle, the you lessen the vacuum or increase the manifold pressure (or manifold absolute pressure (MAP) if you wish) and the engine begins to pick up RPM's or accelerate (which is how the term accelerator derived). This is true whether the engine is a single cylinder Briggs and Stratton or an exotic V16 as long as it is a gasoline/petrol engine (this is not true for a diesel).

Now, the PCM on an LSx series engine matches the fuel flow by regulating the injector duty cycle to maintain the desired air to fuel ratio (AFR). The sensed RPM and MAP are fed to the PCM electronically and are referenced to RPM vs. MAP cells in the VE Table. In Speed Density (MAF disabled) mode, this VE table is the only reference other that the Power Enrichment Table. Also, the mass air flow (MAF) value (if used) is blended with the SD calculation to help get the mixture right.

Regardless of the function of the PCM, any engine will accelerate if you introduce more oxygen into the cylinders and accompany that oxygen with more fuel. The throttle opening initiates the acceleration and the fuel sustains the RPM increase.

Note: with a diesel engine, there is no throttle valve and combustion always takes place with excess oxygen. The fuel flow is metered (or throttled if you wish) to control RPM's. If you introduce more fuel and the oxygen is already there for combustion, the engine will accelerate.

Steve

 

Sounds like you have just taught Engine Mechanics 101. Nice explanantions guys !

 

I used to be a Detroit Diesel mechanic and at age 56, I'm still a gearhead at heart. My day job is as an avionics (aircraft electronics) engineer. I used to be an technical training instructor, but for avionics maintenance technicians and for pilots.

I really like to help people who want to learn to understand things.

All my best,

Steve

 

If a diesel is always running with this excess oxygen, why wouldn't it cause the same problems that occur in a gasoline engine while running lean?

 

That's a good question and I don't know the answer right now. I'll do a little research.

Steve

 

20:1 compression
the fuel doesn't start burning until it's very compressed near TDC.
It also doesn't rely on a spark to start combustion.

If you had to start combustion via a spark at around 30* BTDC you would get the same problems.

 

You are right in that there is no advance for the ignition like with gasoline. With a diesel, the intense heat produced by the 20:1 or so compression means that when the fuel is injected into the cylinder directly (or indirectly via a pre-combustion chamber in older designs) the fuel begins to burn immediately. Also, this beginning of burning begins near TDC. If you want to learn some more about diesels, look at this reference http://en.wikipedia.org/wiki/Diesel_engine. If you want to learn something about a high tech racing diesel slated to run at Le Mans, read this http://www.audi.com/audi/com/en1/exp...sport/r10.html.

Steve

 

But what does it matter how or when it ignites as long as there is all that excess oxygen? I guess what I'm missing here is why does going lean in a gasoline engine cause problems?

 

Interesting discussion.

My auto teacher always favoured us using the terms 'high/ low manifold pressure' as opposed to vacuum.

Vacuum in the truest sense is the absense of matter (air/gas in this case),
and you can never have a perfect vacuum...but rather partial vacuum.

As stated by White2001S10, the gauge reading is a reference to atmospheric
pressure at your current standpoint on earth. If you connect a gauge behind
the throttle blade, you can see the manifold pressure become closer to
atmospheric as the blade opens.

Just remember the gauge reading is not absolute.

As for going lean with a mixture, it creates heat. Depending on the extremes
it will cause the chamber to exceed optimum combustion temperatures causing
knock, or possibly component damage over a period of time.

 

Hummm, this has me thinking. We have always been taught that you can't run a gasoline engine lean.

But here is what I am thinking and someone correct me if I am wrong.
The real reason you can't run a gasoline engine lean is because the fuel burns so fast, that when you start the spark 30 BTDC, the fuel would have expanded to fast and it tries to press the piston down the bore, the wrong way!!!!

So, this would be true all the time. Except, now with electronic timing control, ie, not a distributer, we would be able to reduce the timing at leaner settings and compensate for the quicker burning mixture.

Now, this won't work for emmisions, because it will create more NOx emmissions, I think.

Ok, lets hear what everyone has to say!!!

 

If the timing is adjusted to compensate for an improper mixture, then pressure
on the piston is reduced over the duration of the stroke, and/or the peak combustion pressure is not occuring at the best crank angle.

Result is less power.

 

Yes, you are going to achieve less power, so you will need to open the throttle more, resulting in less pumping losses.
Not sure about the peak pressure occuring at the "right" time, ca50 around 10 degrees ATDC.
I think I need to see some good data on this subject. I am going to do a little research on it.