Probably-stupid engine question

[CTho9305]

Based on some quick googling, it looks like a hemi has a hemispherical cylinder head:
http://www.moraviation.com/images/hemi.jpg

As I understand it, one of the main problems was that it's difficult for the valves to seal with a head like that. Modern engines apparently have something more like a pyramid (?).
http://www.ultimate-racing.com/Services/CylinderHeadPort/CylinderHeadPort-07.jpg

Why don't you just make the piston be hemispherical instead of the cylinder head?
http://ctho.ath.cx/tmp/ctho-hemi.avi

 

[AccruedExpenditure]

F***ing Brilliant. I'm RICH!

 

[Eli]

Originally posted by: CTho9305
Based on some quick googling, it looks like a hemi has a hemispherical cylinder head:
http://www.moraviation.com/images/hemi.jpg

As I understand it, one of the main problems was that it's difficult for the valves to seal with a head like that. Modern engines apparently have something more like a pyramid (?).
http://www.ultimate-racing.com/Services/CylinderHeadPort/CylinderHeadPort-07.jpg

Why don't you just make the piston be hemispherical instead of the cylinder head?
http://ctho.ath.cx/tmp/ctho-hemi.avi

There are dished pistons.

There are also domed pistons.

There has to be some volume in the combustion chamber.. lol.

And no, the problem with hemi heads isn't the valves "sealing", lol....

The combustion chamber design is paramount to engine efficiency. The piston crown shape is dependant on the compression ratio you want to run.

 

[alpineranger]

The picture you posted is of a pent roof combustion chamber, the typical modern design. It allows for a central spark plug location and burns fast, so you can get away with higher compression ratios / lower octane gas. Minimal spark advance is needed (it's not good to rely on too much of this, especially since spark advance must be varied over the RPM range, so a slow burning combustion chamber design would have difficulty in high RPM operation). It'd probaby be pretty difficult to put four good sized valves on a true hemi, at least a lot more difficult than it is on the pent roof design, where intake/exhaust valves are naturally inline.

Why not shape the piston head? Actually, modern piston heads are typically shaped (well- not flat), to allow for additional valve clearance, to promote better mixture swirling, and to control the compresion ratio. (Probably other reasons exist, that I am unaware of). The video you linked to would probably be a poor design for several resons, including: 1) by drastically increasing the combustion chamber volume, you've made major reductions in the compression ratio. 2) with a now spherical combustion chamber, the spark plug will be very far away from some portion of the chamber. This makes a slow buring chamber even slower.

BTW, AFAIK, one of the common criticisms of dodge's new 'hemi' is that it isn't a true hemi.

I really don't know much about cars or things mechanical, so don't take my word as gospel truth - I'm just going off the top of my head here.

 

[Bleep]

higher compression ratios / lower octane gas

I think you have this backwards, the higher the compression ratio the higher octane the fuel has to be. Octane is a number related to how slow the fuel burns, the higher octane the slower the combustion rate. Controlled burn not a explosion.
Wedge shaped combustion chambers work very well and are preferred except for all out racing engines.

Bleep

 

[Triumph]

The point of the hemispherical head is to increase turbulence and swirl of the incoming gas and air. This mixes the two together better and provides for a more complete burn. By making the cylinder head hemispherical, you're basically removing this benefit because the gas and air are injected into the cylinder when the cylinder is at or near bottom dead center.

4 stroke cycle
Intake - cylinder going down
Compression - cylinder coming up
Power - cylinder going down
Exhaust - cylinder coming up

 

[Eli]

Originally posted by: Bleep

higher compression ratios / lower octane gas

I think you have this backwards, the higher the compression ratio the higher octane the fuel has to be. Octane is a number related to how slow the fuel burns, the higher octane the slower the combustion rate. Controlled burn not a explosion.
Wedge shaped combustion chambers work very well and are preferred except for all out racing engines.

Bleep

He meant that due to the combustion chamber design, you can use a lower octane fuel for the given compression ratio than you would be able to with a different combustion chamber design.

Originally posted by: Triumph
The point of the hemispherical head is to increase turbulence and swirl of the incoming gas and air. This mixes the two together better and provides for a more complete burn. By making the cylinder head hemispherical, you're basically removing this benefit because the gas and air are injected into the cylinder when the cylinder is at or near bottom dead center.

4 stroke cycle
Intake - cylinder going down
Compression - cylinder coming up
Power - cylinder going down
Exhaust - cylinder coming up

Cylinders don't move.

Also, air isn't injected. It is drawn into the engine during the aforementioned intake stroke, when the piston is moving down.

 

The point of the hemispherical head is to increase turbulence and swirl of the incoming gas and air.

Incorrect, a bean shaped combustion chamber increases turbulance, any combustion chamber with a large surface area will result in slower combustion and less efficiancy.
The main advantage of a hemi head is flow, that is the only advantage.

As I understand it, one of the main problems was that it's difficult for the valves to seal with a head like that.

Incorrect, it is difficult to design a hemi head without the extreme pushrod angles required to actuate the valves.


Notice that this four valve head's valves are extremely close to the cylinder wall, this will inhibit gas flow hurting performance.

There is a very delicate balance between the following ;

Combustion chamber shape and size
Stroke
Bore
Number of valves
Valve lift and duration
Quench area
Piston shape


Ideally you want a fast burning small combustion chamber with a lot of quench area and with big valves that do not interfere with the cylinder walls, combining all of these is the real trick.


Hemispherical cylinder heads do not have any quench area what so ever, therefor they tend to be inefficiant when compared to bean type or wedge type cylinder heads when it comes to the power to fuel ratio.

 

[Syringer]

Hemi = Dodge != GM

 

[kaizersose]

Originally posted by: Syringer
Hemi = Dodge != GM

marketing = hemi = ~50 year old tech

 

[Iron Woode]

Originally posted by: kaizersose

Originally posted by: Syringer
Hemi = Dodge != GM

marketing = hemi = ~50 year old tech

50 year old tech that was 30 years ahead of its time.

Roger is correct:

The Hemi's (426) main advantage was huge valves placed as best as possible and a centrally located spark plug. This meant huge flow and big power numbers.

The biggest disadvantage was both port design and rocker geometry. Port design was compromised because the engine had to fit into a B body chassis. The way they wanted to do the heads would have required the car to be much wider. Hemi exhaust ports face downward and have convoluted passages impeding flow.

The valvetrain was a whole new ballgame as it seemed very complicated. Unless they went OHC, Mopar had to live with some pretty strange pushrod angles and oddly designed rocker arms. The system worked but needed improvement.

 

[Howard]

Hemis burned pretty damn slowly... Advance the spark... more... more... more... almost there! BANG

 

[Howard]

Originally posted by: Roger

The point of the hemispherical head is to increase turbulence and swirl of the incoming gas and air.

Incorrect, a bean shaped combustion chamber increases turbulance, any combustion chamber with a large surface area will result in slower combustion and less efficiancy.
The main advantage of a hemi head is flow, that is the only advantage.

As I understand it, one of the main problems was that it's difficult for the valves to seal with a head like that.

Incorrect, it is difficult to design a hemi head without the extreme pushrod angles required to actuate the valves.


Notice that this four valve head's valves are extremely close to the cylinder wall, this will inhibit gas flow hurting performance.

There is a very delicate balance between the following ;

Combustion chamber shape and size
Stroke
Bore
Number of valves
Valve lift and duration
Quench area
Piston shape


Ideally you want a fast burning small combustion chamber with a lot of quench area and with big valves that do not interfere with the cylinder walls, combining all of these is the real trick.


Hemispherical cylinder heads do not have any quench area what so ever, therefor they tend to be inefficiant when compared to bean type or wedge type cylinder heads when it comes to the power to fuel ratio.

Larry Widmer does it pretty damn well.

 

[Bulldog13]

I changed my own oil once.