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The Double acting 4x4 single cylinder steam engine

Here's a complete construction sequence for a LEGO engine. This is a real smooth runner, and being double acting, it can start from almost any degree of the crankshaft's rotation. I wish I had the LEGO rpm sensor to test it for sure, but from the sound it makes, my guess is that it reaches 700 rpm under full (and I do mean full) 'steam.'

This page is quite graphic intensive. So those of you still on 33kbps modems, please be patient.

If 700 rpm isn't impressive enough for you, then check out Mecevan's page. With the lego rpm sensor, this fellow has been getting 2300 rpm readings out of a similar engine to mine, running on suction from a vacuum cleaner.

The cylinder head, a 6x6 plate

This engine uses a 6x6 cylinder, and a 4x4 piston. I attribute the higher speed with this design, it would seem that the added displacement helps to offset the friction. It seems there's not the same increase in friction in the larger size. (Some of my other engines used 2x2 piston designs.)

Parts:
• 6x6 plate x 1

The cylinder head, shows top intake port

Be sure to get this in the right spot. The valving/timing depends on the location of the ports.

Parts:
• 6x1 brick x 2
• 4x1 brick x 1
• 2x1 brick x 1
• 1x1 brick x 1

The cylinder head, second row

Parts:
• 6x1 brick x 2
• 4x1 brick x 2

The cylinder head, third row

Parts:
• 6x1 brick x 2
• 1x1 brick x 4

Cylinder pivots

The cylinder will 'rock' on these pivots. I used 2x1 blocks with the pegs molded right into the side. You could also use a 2x1 block with a hole, and then one of those grey pins, but the mechanism would be a little more sloppy.

Parts:
• 2x1 brick with pegs x 2

Pivot blocks

Parts:
• 2x1 brick with holes x 2

Piston and rod assembly

Use a 4x4 plate for the piston, and a gear snaps right into place in the center. Then a 6-long shaft is used for the connecting rod.

Parts:
• 4x4 plate x 1
• 24 tooth gear x 1
• 6 length shaft x 1

Piston fitted into cylinder

Base of cylinder

Parts:
• 4x2 plate x 4

Connecting rod exit gland

Parts:
• 4x2 plate with holes x 1

Plugging the extra holes

Parts:
• 1x1 plate x 2

Wrist on connecting rod

Parts:
• wrist

First row of engine base

Parts:
• U figure it out

Second row of engine base

Engine base, note notches for pivot supports

Engine base, pivot supports in place

Parts:
• 2x2 inverted slant brick x 2

Engine base, note notch for crankshaft bearings

Parts:
• U figure it out

Crankshaft and bearings in place

Parts:
• 2x1 brick with holes x 2
• Grey shaft collars x 2
• 8 length shaft x 1

Bearings completed, crankwheel and flywheel in place

Parts:
• 40 tooth gear x 1
• Large tire x 1

Cap on top of bearing assembly

Parts:
• 4x3 slant brick x 2
• 4x2 peak brick x 1

Cylinder put into place

Parts:
• Grey pin x 1
(Attaches connecting rod wrist to crankwheel)

Final row on engine base

Ok, ok. I've been saying 'U figure it out' on these steps. That's because there's an almost infinite number of possible ways to fill in these parts. However, I will say this: Make sure you interlock the parts with the layer below! In other words, cover the seams so that there are as few 'stacked seams' as possible, so the structure is solid and strong! (You expert LEGO builders out there know what I'm saying.)

Steam chest bottom is 4x8 plate

Parts:
• 4x8 plate x 1

Steam chest porting

Parts:
• 4x1 brick x 4

Completed engine!!

Parts:
• 4x8 plate x 1

As a final step it may be necessary to put the engine on a nice flat surface and press firmly around the engine base (and the cylinder before mounting it in the engine base). This more thoroughly aligns the whole engine and reduces binding. You may also need to do this periodically as well, because the parts tend to work loose with running.

Also, a word or two about running. The engine will not start at top (0°) or bottom (180°) dead center. It needs to be slightly past one of these points for the piston to apply sufficient starting torque. The engine may also need a stronger starting blast of air than it does to keep it running.

There seems to be something about the way a person blows, too. Some of my friends are completely unable to run the engine; they just slobber all over it. Others (including myself) know something about puckering up and giving a good hard 'dry' blow into the engine. If all else fails, and you are certain your engine spins freely, try running it backward with a vacuum cleaner. This is how like to I like to run my double acting twin cylinder engine.