The Engine Shop — 2010/08
Enya's diesel four-stroke
Joe Wagner | [email protected]
My mention of Enya engines in a recent column brought a quick reply from David Pawlowski of Midland, Michigan. He informed me that Enyas are readily available from their Japanese manufacturer — Enya Direct — and with amazingly fast delivery. I checked this out and began a lengthy and highly educational e-mail correspondence with Ken Enya.
One thing I was especially curious about was Enya's four-stroke diesel. Several of the four-stroke model engines I've tested and reported on had remarkably snappy compression, and at those times I wondered whether anyone had ever made a diesel version of one of these. It turns out Enya has, and Ken sent me one to try. It's a .41 and is made with Enya's typical high quality and ingenious design.
The engine is based on the company's glow 41-4C four-stroke and uses most of the same parts. Its head even has a threaded hole for a glow plug, but in the diesel version this is filled with a plug machined from solid metal. It looks like a glow plug, though.
Unboxing and assembly
When I opened the 41-4CD box, I received quite a surprise: the engine is sold only partly assembled, with many small parts supplied in individual plastic envelopes. Ken told me he believed anyone who bought one of his four-stroke diesels would most likely be a model engine enthusiast—someone who would enjoy seeing the guts of this unusual little power plant before putting it to use.
Assembly and operating instructions that came with the 41-4CD were skimpy and imperfectly translated into English. However, with decades of experience with model engines I had no real trouble putting the Enya together. A full set of assembly tools comes with the engine.
Key assembly steps I followed:
- Remove the valve rocker assembly. It is attached with a single long screw and comes off as a complete unit that needs no readjustment when reattached.
- Pull out the pushrods and put a few drops of castor oil into each cam follower hole to lubricate the cams and gears. Add a drop of oil to each rocker pivot as well.
- Loosen the four head screws. It is unnecessary to remove the head completely; loosen it only enough to allow the pushrod tubes to be inserted. (I removed the head to inspect the combustion chamber.)
- Insert the pushrod tubes, drop the pushrods into their tubes, and reassemble the top end.
- Tighten all five head-retaining bolts progressively to seat the head evenly — as tight as you can without permanently bending the Allen wrench. Then add the valve cover.
- Remove the carburetor before reinstalling the intake tube so you can line the tube exactly and ensure no leakage at either end. Once the intake tube is properly aligned in both carburetor and head, reattach the carburetor and add the muffler assembly.
The engine ships with extra head gaskets. These are needed for hot-weather operation.
Compression, fuel, and overheating
The two .0045-inch-thick gaskets that came installed in my engine provided proper compression for easy starting and smooth running at about 70°F. At 88°F I saw sure signs of overheating: black oil in the exhaust. I took the top end apart and installed one extra gasket, which fixed the overheating problem.
I judge the correct compression setting for all my diesels by the color of their exhaust oil. I try to keep it light — preferably no darker than iced tea. Black diesel exhaust oil indicates the mixture is firing well before the piston reaches top dead center (TDC). That leads to excessive heat, loss of power, and extra strain on the wristpin and connecting rod. Reducing compression cures those issues and yields smoother running and more useful power.
Note: model diesels aren't true diesels because they lack fuel injection at the top of the piston stroke. Model "diesel" engines are more accurately called compression-ignition types. Instead of ignition by spark or a glowing coil of platinum wire, the heat generated by high compression at TDC causes the ether in the fuel to combust, which ignites the kerosene. Typical model diesel fuel composition is roughly: ether ~35%, kerosene ~40%, castor oil ~25%.
Nearly all model diesels feature adjustable compression, which permits using high compression while starting and then reducing compression as the engine warms up. That feature makes model diesels flexible; most can happily spin any size propeller that will fit on the shaft — an .03 diesel can even turn a 10 x 4, believe it or not.
However, the Enya 41-4CD does not have adjustable compression. It is similar to the first American-made model diesels (the 1947 Drone .29 and the Mite .09). Those older engines ran fine in cold weather but became balky and rough-running in summer as they fired prematurely when warm. That doesn't seem to be a serious problem with the Enya 41-4CD: its efficient four-stroke operation and extensive cylinder and head fins keep its operating temperature reasonably cool.
Starting, props, and running impressions
Enya recommends hand-starting the 41-4CD. Electric starting isn't entirely safe for any model diesel because high compression makes "hydraulic lock" a distinct possibility. The 41-4CD's piston comes so close to the top of the combustion chamber at TDC that the piston's crown has two semicircular pockets machined into it to provide valve clearance. Only a little liquid fuel in that space would be enough to stop the piston before TDC and cause a bent rod or worse during electric-start attempts.
Enya recommends propellers from 12 to 14 inches in diameter and 5 to 8 inches in pitch for the 41-4CD. I chose an APC 13 x 6 to begin with. It's heavy enough for good flywheel action — helpful for a diesel and essential for a four-stroker.
Instructions call for finger choking before starting, but I find that awkward on upright four-strokes. Instead, I block off the muffler exhaust outlet with a fingertip. Flipping the propeller with the exhaust sealed pushes pulses of muffler pressure to the tank to force fuel into the carburetor. As soon as the engine fires, I remove my finger from the muffler port and continue flipping vigorously until the engine starts.
The 41-4CD fired quickly. I left the mixture rich at first, with clouds of white smoke pouring out of the exhaust to maximize internal lubrication during the initial minutes. When I first turned it over with a prop installed, the engine felt almost as snugly fitted as a glow ABC (aluminum-brass-chrome).
After 40 minutes of running at various rpm the 41-4CD still feels very snug. However, it gained about 200 rpm at the top end (now 7600 vs. 7400 initially) and its reliable idle speed reduced from 2600 when new to 1900 now. Later, when I tried an APC 12 x 6, performance improved even more.
Cautions and tips
- Two standard diesel cautions apply:
- Never flood it. Flooding prevents starting and makes it hard to crank a diesel dry because a diesel burns substantially less fuel than a glow engine — at least one-third less. I flooded the 41-4CD once and suffered the consequences.
- Never idly flip a diesel's propeller. It can start.
- For hot-weather use, add head gaskets as needed to reduce compression and prevent overheating (as judged by exhaust oil color).
Overall impression
I like Enya's four-stroker. It's ruggedly made, easy to start and adjust, and runs as smoothly as any four-stroke I've handled. Its performance on the APC 13 x 6 surprised me for a .41, but model diesels are renowned for their ability to swing big propellers.
ProGlo starter: maintenance note
I've used Ni-Cd-powered glow plug lighters for decades and always hated replacing the entire unit when the battery died. Tower Hobbies' ProGlo Starter is different: its sub-C battery is as easy to replace as one in a flashlight.
A couple of months ago I noticed my ProGlo wasn't working well and took a long time to recharge, yet the Ni-Cd tested fine. Inside I found engine oil had worked its way into the interior and formed a thin, high-resistance film between the electrical contacts. The cure was to clean everything with grease-dissolving detergent and lightly sand all contacting surfaces, including the ends of the Ni-Cd. That fixed the problem.
My ProGlo has the optional metered top. It's not a precision meter, but it makes a good diagnostic tool: it instantly detects a blown plug and lets you know when the Ni-Cd needs recharging. The time saved using the metered ProGlo has made it well worth the extra $10.
Sources
- Enya Direct — www.enya-engine.com
- Tower Hobbies — (800) 637-6050, www.towerhobbies.com
Transcribed from original scans by AI. Minor OCR errors may remain.
