How a torque converter increases the torque of a Diesel engine?

by increasing the diameter of the gears 

Hi Shiv

I will answer the question in the best possible way I can. The torque converter is basically a clutch on an automatic transmission. As the clutch transfer the torque from the engine output i.e the flywheel to the drive train a Torque converter transfers the torque to the drive train(transmission). A clutch transfers the torque from an engine, as it is as soon as it contacts, there is no increasing ratio, but in a torque converter there is a variable ratio, possibly infinite torque ratio transfers. The Torque converter as three parts one is a impeller, turbine and a stator. The impeller receives the output from the engine an throws the fluid to the turbine. If you keep two fans opposite to each other one rotating and other stationery, the stationery fan rotates in the direction of the rotating fan. similar principle, but here we have the stator which multiplies the torque by throwing back the fluid in torque converter onto the impeller thereby multiplying the torque. This is a quick and efficient way of transferring the torque without many wearable parts. the output from the turbine is transferred to the transmission. the multiplication of torque and transfer in the Automatic Transmission is another completely different method. Hope this answer clears your doubt. The stator in the middle of the torque converter is the one which multiplies the torque.