cylinder pressure 1

[Greeno1645]

Okay, here's a softball question that I'm sure everyone but me (who knows nothing about combustion engines) can answer.

What is the PSI of a cylinder in a 4,6, & 8 cylinder engine?

The reason for the question stems from my curiousity into building an electric engine that operates like a combustion engine. Instead of igniting gasoline to create pressure, use a strong electro-magnet in the engine block with a fixed magnet in the cylinder - an alternator can be used to change to polarity of the current, either to repulse the cylinder or attract it. And, current between the individual cylinders could rotate (firing order) just as they do in combustion engines.

Anyone want to take a stab at how large an electro-magnet would be needed to create the same amount of PSI?

 

[BrianPetersen]

The number of cylinders has no influence on the peak cylinder pressure. The compression ratio, intake manifold pressure (particularly in the case of turbocharged engines), volumetric efficiency, and the timing of ignition (spark) or injection (diesel) events affect the peak cylinder pressure. Some production automotive diesels have peak cylinder pressure in the 180 bar range.

By the way, your proposal will result in an extremely complicated and expensive electric motor which is probably less efficient than a standard electric motor, and I don't see the advantage over a plain ordinary electric motor, but if you know something that I don't, good luck with it.

 

[GregLocock]

Designing an efficient variable air gap electric motor is going to take a while.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EdDanzer]

What you are trying to do is similar to what is needed for a linear engine. If you do a search on Google for “linear engine” and “free piston generator” you should find a NASA engine, other engines, and many papers about development. The difficulty in linear electric motor/generator design is the range of speed, power required is great. The speed and force at ignition is very high, yet close to the bottom of the stroke, speed and pressure are low. 2 stroke engines will probably never meet pollution requirements so a 4 stroke design will require the motor/generator to drive the pumping cycle of exhaust, intake, and compression. The efficiency of driving the pumping cycle must be very high. Intake and exhaust don’t need much power, but compression does require quite a bit depending on compression ration and bore size. Since the pumping energy must be stored as DC this complicates the problem even more.

 

[IceStationZebra]

This would be a very inefficient way to create torque. If you generate 200psi on the way up you are left with the same 200psi to drive the piston down - no net gain. Plus take out the energy lost to friction, heat, air flow losses, etc. You will only get back a small portion of the energy you put in, I would guess in the 50% range at best. Every time you transform energy some is lost to inefficiencies - battery(chemical) to electrical to magnetism to linear mechanical motion to compressed air to linear mechanical motion to rotational mechanical motion.... A google search shows that traction motors are ~90% efficient. What do you see as the benefit? ISZ

 

[patprimmer]

One fairly large problem will be that the strength of a magnetic field will be inversely proportional to the square of distance from the source. For this reason, the field and armature magnets in a normal DC electric motor are placed as close to each other as possible while ensuring they do not actually hit.

Also you do not make it clear if you intend to open and close the valves on the normal engine.



Regards

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[soros151]

How about a magnetic drivetrain in every wheel with no engine. Use polarities to go or to stop and somehow use brake energy to recharge the batteries. Maybe I'm just rambling my head out in fantasy land, go on please.

 

[patprimmer]

It's been done, about 100 years ago by Ferdinand Porsche.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BrianPetersen]

They're called "wheel motors", they have been under investigation many times before, and nowadays, they are generally conventional (rotary) AC motors driven through variable-frequency drives. No need for simulating an internal combustion engine ...

 

[GregLocock]

More accurately they are brushless DC motors with electronic commutation.

But they are on the blurry edge of being 3 phase synchronous AC motors.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Greeno1645]

Everyone has given very valuable advice and information. Thank you. I'm more of a 'concept' guy than a numbers and calculations guy.

That said, the idea stems from the concept of being able to create an electro-magnetic engine in the shape of an ordinary combustion engine so that it could be installed into any gasoline driven automobile without modifying the other components of the vehicle AND since the design operates with magnetically driven pistons (and valves!) it would (or could) have a similar sound and appeal of a combustion engine to those who like 'classic' designs.

The vision is that when you step on the gas, the engine revs by increasing the speed of the pistons action by increasing the magnetic force of the electro magnet.

Impractical - yes, no doubt about it when compared to just putting electric motors behind each wheel. However, if the engine can be built from ceramic or some type of light weight/strong polymer, then perhaps by lessening the weight of the vehicle one can compensate for loss of performance from the lack of efficiency.

So, the question remains: How strong a electro magnet would one need to create the type of pistion movement that takes place in a combustion engine?

 

[GregLocock]

To be done badly it is easy. Any clown with a magnet and a piece of wire could do it.

As we have tried to intimate, to do it well is very, very, difficult, perhaps impossible.

So, are you a clown, or are you willing to listen?


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IceStationZebra]

You are going through a lot of effort to create vibration and noise, and it won't sound anything like an internal combustion engine. And your overall efficiency will be half of the average car - all when gas is headed toward $4/gallon!

"could be installed into any gasoline driven automobile without modifying the other components of the vehicle" - IMPOSSIBLE!
- engine installations often vary from year to year within a given model, so you would need thousands of different configurations.
- What about heater/defrost?
- If the weight varies by much the handling, antilock braking and stability control will be compromised.
- Where are you going to get all of the power needed to drive the magnets? Are you going to hide a dozen batteries in the truck? I haven't seen any dilithium crystals for sale yet.
- Every automatic transmission will need to be retuned, with most requiring custom tweaked software.
- etc., etc.

I like to think outside the box, but this is a lost cause.
ISZ

 

[Greeno1645]

Okay....I surrender.

Pull the plug - this one is DOA.

Thank you.

 

[drwebb]

Okay guys, congratulations on a thorough design grilling to an admittedly strange idea, but you haven't gone far in answering the core engineering question: How big of an electro magnet or field would be required to accelerate a piston comparable to an internal combustion event? It is a bit of a curiosity . . .

 

[SomptingGuy]

I think patprimmer gave the most important technical answer here. The physics are different. The force/displacement behaviour of a magnetic system is very different from a compressible gas system. So there is no real comparison.

 

[EdDanzer]

To answer the question more accurately you would have to specify the bore size, compression ratio, fuel type, and fuel volume as a minimum. Piston speed can have and effect as well at temperature and humidity. For a 90 mm gas engine at 9:1 compression the force can be over 10,000 pounds for a short time.

 

[dicer]

If you wanted a good place to start, you could use 150psi BMEP.