Turbo-charged diesel engines offer advantages in terms of high output power, but are of higher initial cost. Problems exist with respect to matching of the turbocharger with engine at part loads .As a cheaper alternative to such engines, reciprocating compressors in-built in a multi-cylinder engine functioning in lieu of a turbocharger unit can be built.This arrangement can be done either as a retrofit or as an ab-initio option.

In this engine some of the cylinders function as reciprocating compressors while others act as power producing cylinders . Fig.1 depicts a typical arrangement for a six-cylinder inline engine. In this six cylinder engine , 3 alternate cylinders function as reciprocating compressor ( C1,C2,C3) and three as power producing cylinders ( E1,E2,E3) .The three cylinders are phased at 120 degrees with respect to their stroke positions in order to have symmetry / uniformity of output and pressures .The preferred firing order is E1-E3-E2. When the piston of E1 is at TDC, the piston of C3 is at BDC i.e when E1 is about to fire, C3 is about to compress the air. The compressed air output from cylinders C1,C2 and C3 are connected to an air receiver of suitable capacity.The three power cylinders draw their inlet air from the air receiver operating at higher pressures and hence the peak cylinder pressures will also be high.

In naturally aspirated engines, the inlet and exhaust valves open once in 2 revolutions of crank shaft.In this engine the compressor inlet and outlet valves need to open in every crankshaft rotation .In addition since only 3 power cylinders are operating now, the relative time of opening of valves change and these require cam shaft re-design.Existing engines can be readily retrofitted to convert them into this configuration. The cylinder head requires a new design for incorporating various ports.. In retrofit engines normally operating without turbo-charging, in order to moderate their high peak pressures, the compression ratio of the firing cylinders may have to be reduced slightly, by altering their piston designs. Their operating pressures would still be higher than their original design pressures and to this extent, safe working pressures of cylinders need to be checked and complied with.

In new designs, however maximum benefit can be realized by increased operating pressures offered by turbo charging using the reciprocating compressors. The pressurised air receiver can also be used for atomizing the fuel in air-blast atomizers.