Usage:
1-1 Engine Proper
(1) Combustion Chamber
(a) 4D31, 4D31-T
The combustion chamber consists of the cylinder head and toroidal pistons, hole type injection nozzles being installed in the cylinder head.
Combustion occurs when the fuel is directly injected into the combustion chamber with the power operating the piston directly.
(b) 4D30, DR
The combustion chamber is formed by the lobes shaped main combustion chamber in the piston head and Mitsubishi's own bell-shaped swirl chamber.
A chamber jet is press-fitted into the swirl chamber that is made of material with little thermal expansion and good thermal conductivity. Also installed in the chamber are the sheathed glow plug and throttle injection nozzle.
The upward movement of the piston during the compression stroke imparts a violent swirling motion to the air charged into the swirl chamber, to which fuel is injected for combustion. As the combustion goes on, the gas moves from the swirl to main combustion chamber where it is mixed with the air present. And further combustion occurs.
For more effective cooling of the combustion chamber, water directors that direct coolant flow are pressed into the bottom of the cylinder head.
(2) Valve Mechanism
The valve mechanism is an overhead valve type.
(a) Both the inlet and exhaust valves are made of surface-treated heat-resistant steel. The valve seat angle is 45° in both valves.
Valve stem seal is mounted to the valve stem, which controls the amount of lubricant on the sliding surfaces between the valve and valve guide.
A valve guide with carbon cutter is used for the exhaust side.
(b) Two unevenly pitched valve springs are used, the inner and outer with coil directions opposite to each other.
(c) The rocker has an induction-hardened surface in contact with the valve cap. A bushing is used in the hole, into which the rocker shaft is fitted. Because of them, wear resistance is improved.
(d) The rocker shaft is a hollow round rod with sealing caps at both ends to seal it off. Its inside is an engine oil passage.
(e) The push rod has a steel ball welded to its bottom end and a spherical or depressed piece welded to its top end. Both ends are carburized case-hardened.
(f) The tappet on the D3 is of cylindrical shape, while that on DR of mushroom shape. Its mating surface with the camshaft is a large-diameter sphere to prevent local wear.
(g) The camshaft is provided with an oil pump drive gear, with its journals, cams, and gear induction-hardened. The cam on D3 is a taper cam.
(3) Crankcase and Cylinder Sleeve
(a) The crankcase is made of cast iron and built rigid with minimum stress concentration and deformation.
(b) Three camshaft bushings (four on 6DR5) are installed in the camshaft bearings of the crankcase. To facilitate insertion and removal of the camshaft from the front end of the crankcase, the bushing I.D. is made smaller toward the rear.
(c) The special cast-iron cylinder sleeve, that is highly resistance to wear, is press-fitted into the crankcase.
(4) Piston and Piston Ring
(a) Piston
The piston, made of aluminum alloy castng, has a strut inside (no strut installed on 4D31-T) that ensures an adequate clearance between the piston and cylinder sleeve.
The piston-pin arrangement is full-floating. The piston pin is offset toward the thrust side, which, together with the effect of the strut, minimizes piston slap.
Stamped on the top surface of the piston is the piston weight mark, plus the front mark (triangle mark) on 4D31 and 4D31-T.
On 4D31-T, an ID mark "T" stamped on top and a Niresist piston ring insert is cast in the top ring groove for increased durability.
(b) Piston ring
There are three piston rings installed: two compression rings (three on DR) and one oil ring. The sliding surface of each ring, except for the 2nd and 3rd compression rings on DR, is hard chrome plated.
Piston rings are shaped as illustrated.
(5) Connecting Rod and Connecting Rod Bearing
The connecting rod is a die forging with an I cross section ensuring high rigidity. A lead bronze bushing is press-fitted into its small end while a split-style plain bearing is used in its big end.
On D3, an oil passage is provided obliquely through the stem and an oil jet at the small-end to lubricate the small-end bushing and cool the piston. The connecting rod small end of 4D31-T is wedge-shaped.
(6) Crankshaft and Main Bearing
(a) Crankshaft
The crankshaft is a highly-rigid die forging integral with the balance weights. The pins, journals, and rear oil seal sliding surfaces are induction-hardened for improved wear resistance.
By means of oil passages drilled through the pins and journals, part of the main bearing lubricating oil is fed to pins for lubrication of the connecting rod bearing.
At the front end of the crankshaft, there are a crankshaft pulley and crankshaft gear attached with a nut. The crankshaft pulley drives the alternator and water pump through the V-belt and the crankshaft gear drives the camshaft and injection pump.
On 6DR5, a torsional damper may be installed that absorbs torsional vibration of the crankshaft.
The flywheel is mounted in the rear of the crankshaft.
An oil seal is installed at front and rear of the crankshaft.
(b) Main bearing
The main bearing is a split-style plain bearing made of special alloy plated kelmet metal with a steel back.
The upper main bearing has an internal oil groove and oil hole which is aligned with the oil hole in the crankshaft.
Five pairs of main bearings (seven pairs on 6DR5) are used. Split-style thrust plates are mounted with the rear pairs of main bearing.
(7) Timing Gear
The timing gears are housed in the timing gear case at the front of the engine. Illustration at right shows the gear train.
Each gear is a helical gear machined by a shaving machine to high precision and surface-treated for enhanced durability.
A timing mark is stamped on each gear. At reassembly, correct meshing can be achieved by aligning these marks.
A bushing is press-fitted into the idler gear which turns on the idle shaft. The oil hole drilled through the idler shaft and gear provides an oil passage for lubrication of bushing and gears.
On 4D31 and 4D31-T, an oil pipe is installed in the for forced lubrication of the injection pump gear.
(8) Flywheel
The flywheel is made of cast iron. The pilot bearing of the transmission drive pinion is installed at its center. On its periphery, the ring gear is shrink-fitted that meshes with the starter pinion.
The ring gear tooth crests are induction-heardened for greater durability. At the same time, one side of the crests is chamfered to ensure that the starter pinion meshes easily when starter is operated.
1-2 Air Compressor <Option: 4D31>
The air compressor is installed on the flywheel PTO gear cover at the rear of the engine and is driven by the crankshaft that is in mesh with the PTO shaft.
The piston is made of aluminum alloy and carries two compression rings and one oil ring.
The bearings are lubricated by engine oil supplied from the oil main gallery through oil pipe and oil hole of the crankshaft. The cylinder liner and the connecting rod small end are splash lubricated by rotation of the crankshaft. Excess oil is returned to the oil pan from the crankcase end through an oil pipe and the PTO gear case.
The cylinder head of the air compressor has valve seat, delivery valve and unloader valve installed.
When the piston goes down, the resulting negative pressure causes the valve plate to open, allowing air to be introduced in the cylinder. When the piston goes up, the valve spring closes the valve plate, compressing air.
The air compressed by upward motion of the piston overcomes the valve spring force to open the valve plate and is fed to the air tank.
The unloader valve acting with the pressure governor adjusts the pressure in the air tank. When the air pressure in the air tank reaches the specified pressure (pressure governor operating pressure) or higher, the pressure governor is opened to supply compressed air to the unloader valve to force it down. The compressed air also keeps the valve plate pushed open so that the air compressor is now run unloaded.
When the air pressure in the air tank drops to below the specified level, the pressure governor operates again to release compressed air from the upper part of the unloader and as a result, the unloader valve is made to return to the original position by the return spring. Hence, the valve plate starts to operate.
1-3 Flywheel PTO <Option: 4D31>
The flywheel PTO is installed on the flywheel housing at the rear of the engine. Power is taken from the crankshaft gear mounted to the rear end of the crankshaft and transmitted from the idler gear No. 1 to idler gear No. 2 and then to the PTO gear.
