Ajax Compressor Troubleshooting: A Field Guide for the Integral Engine Compressor

One machine, two jobs: how the Ajax integral works

The Ajax integral engine compressor, built by Cooper (Ajax), is a slow speed machine where the power cylinders (the engine) and the compressor cylinders sit on the same frame and turn the same crankshaft. The older E series and the DPC 2801 through DPC 2804 all follow this layout, and the model number often tracks the count of power cylinders. Ratings run from roughly 148 to 845 brake horsepower at conservative speeds near 300 to 440 rpm.

The engine is two cycle. It has no intake or exhaust valves, no camshaft driven valve train, and no turbocharger. Ports in the cylinder wall handle intake and exhaust, and a scavenging box pressurizes fresh air to about 10 psi before the piston uncovers the ports. That simplicity is why the Ajax lasts for decades, but it also means the few parts that do wear matter a lot.

Because one crankshaft drives everything, a problem in the engine and a problem in the compressor can produce the same complaint at the panel: rough running, low flow, or a shutdown. Good troubleshooting starts by deciding which half of the unit is actually at fault.

Power cylinder misfire and ignition

A misfire on a power cylinder shows up as rough or uneven running, a drop in power, a wider exhaust temperature spread between cylinders, and sometimes visible smoke. On a two cycle Ajax there is a power stroke every revolution, so one dead power cylinder drags the whole unit.

Common causes

• Fouled or worn spark plugs. Oil or carbon fouling shorts the plug to ground. Check the gap, and treat oil fouling as a clue, since it can point to worn rings or too much cylinder lube.
• Weak or lost spark. Ajax units use an Altronic solid state ignition with very few moving parts. Check the lead, the coil, and the low tension wiring before you condemn the ignition unit.
• Ignition timing. The Altronic system is timed at the factory and rarely drifts, but a disturbed pickup or a wiring error can throw one cylinder off.

To diagnose, pull and read each plug and compare exhaust temperatures cylinder to cylinder. A single cold cylinder that still has a good plug points to a fuel or compression problem on that cylinder, not the ignition.

Fuel gas and injection problems

The Ajax uses a cam gear actuated hydraulic fuel injection system with a rotary gas valve, so fuel delivery is mechanically timed to each power cylinder. Fuel faults are one of the most common reasons for hard starting, low power, and surging.

What to check

• Fuel gas supply and pressure. Confirm the supply pressure and that the regulator holds steady. Wet or dirty fuel gas fouls injection parts and plugs.
• Injection check valves and injectors. A stuck or leaking fuel injection check valve delivers the wrong amount of gas to a cylinder, which reads as a misfire or uneven load.
• Gas quality. Heavy or liquid laden gas changes combustion and can wash the oil film off the cylinder walls.

Because injection is timed by the cam gear, a worn injection cam or linkage can shift timing on one cylinder. Trend the per cylinder exhaust temperature to find the odd cylinder before it starts knocking.

Compressor valve failures

On the compressor end, the suction and discharge valves are the parts that fail most often. A leaking valve lets gas slip back past it, so the cylinder does less useful work and heats up.

Symptoms

• Falling flow or throughput at the same speed.
• High discharge temperature on one cylinder, often the clearest early sign of a leaking valve.
• A valve cap that runs hotter than its neighbors.
• Knocking or unusual noise as a valve plate or spring breaks up.

Confirm with a PV card (pressure volume analysis) if you have the gear, or with valve cap temperatures as a simple field check. Valves fail in a predictable order: springs weaken first, then sealing elements leak, then plates crack. Catch the leak early so a broken plate does not score the cylinder bore.

Rod packing, piston rings, and low compression

Rod packing seals the piston rod where it passes through the pressure packing case on the compressor cylinder. Worn packing leaks gas along the rod and costs you capacity.

• Signs of packing wear: gas escaping to the distance piece or vent, a wet or oily distance piece, higher packing temperature, and lost flow.
• Common causes: normal wear, poor lubrication, a scored or bent rod, or dirt in the packing case.

On the engine side, low compression in a power cylinder comes from worn piston rings, a scored liner, or a leaking cylinder. It shows up as hard starting, low power, and oil fouled plugs, and a compression or leak down check will find it. Keep the force fed cylinder lubrication working, meaning the divider block and every lube point, because a dry cylinder wears rings and liner fast.

Cooling, bearings, and timing on a slow speed integral

Thermosiphon cooling

Many Ajax units cool by thermosiphon. Hot water rises and cool water falls by natural convection, so there is no water pump and no thermostat. That is simple and reliable, but it also means low coolant, scale, or a trapped air lock stalls the flow and the unit overheats quickly. Keep the coolant full and clean, vent air after any service, and watch jacket water temperature.

Bearings and lubrication

Main and connecting rod bearings run on a splash lubrication system, so there is no oil pump, filter, or cooler on that circuit. Watch oil level and oil condition, and listen for a knock that signals bearing wear. Babbitt sleeve or tapered roller bearings carry the crankshaft, and running rod load beyond the frame rating shortens their life.

Timing and the whole unit

Because the engine and compressor share a crankshaft, ignition timing, injection timing, and compressor loading all interact. Keep loading inside the frame rod load limit, leave timing at the factory setting unless a fault forces a change, and adjust one thing at a time when you tune.

A simple maintenance rhythm for the Ajax

A slow speed integral rewards steady attention. Most failures give weeks of warning if someone is watching the numbers.

• Daily: check oil level, coolant level, and jacket water temperature, and listen for any new noise.
• Each service: read spark plugs, check the exhaust temperature spread, inspect fuel injection parts, and confirm the divider block is feeding lube to every point.
• Periodically: pull and inspect compressor valves, check rod packing and rider bands, and log rod load.
• At overhaul: inspect bearings, liners, rings, and the crankshaft. The sealed crankcase design stretches oil change intervals, but do not stretch the inspections.

Trend the readings over time. Exhaust temperature spread, discharge temperature, vibration, and flow will almost always drift before something breaks.

How EverSense helps you catch Ajax problems early

The hard part on an Ajax is telling the engine half from the compressor half, and catching a slow drift before it becomes a broken valve plate or a scored liner. EverSense is predictive maintenance software for gas compression fleets. It reads your existing service data, exhaust and discharge temperatures, run hours, and repair history, then flags the cylinder or system that is trending wrong and drafts the likely cause. Every diagnosis is checked against the unit's own OEM manuals and a repair archive built from about 25,000 field reports across 38 equipment makes. It is advisory only, so your crew always makes the final call. If you run Ajax units, or a mix of integral and separable machines, book a demo to see it read your own data.