Turbo lag is a very common phenomenon with all diesel engines using the turbo charger to boost power output from an engine. Naturally aspirated diesel engine which is more polluting, heavier, having higher power losses makes a diesel engine more lethargic. Turbocharged diesel engine is fuel efficient, having lower emissions and better power. A smaller sized turbocharged diesel engine delivers power equivalent to larger sized engine;
Turbo Lag is the time required to change power output in response to throttle inputs. Turbo lag results in slow increase of speed when we press the accelerator pedal. Turbo lag becomes a real cause of concern when rapid changes in power are required. This is due to the time required for a turbocharger and exhaust system to generate the required boost. Inertia, Friction and Load on the compressor are prime contributor of Turbo Lag;
Applications of Turbocharger can be sub-categorized into those that require frequent changes in power output such as automotive and those that do not require frequent changes such as locomotives, aircraft, marine, commercial automotive, gensets, and locomotives. Turbo lag is more prominent and problematic for diesel engines require rapid change in power output. Engine designs reduce Turbo lag in a number of ways:
Changes in turbine aspect ratio;
By reducing bearing frictional losses using a foil bearing in place of conventional oil bearing;
By using lighter and smaller parts which will reduce rotational inertia of the turbocharger;
By using variable-nozzle turbochargers;
Using multiple turbochargers sequentially or in parallel;
By increasing compressor discharge pressure and waste-gate response;
Using a turbocharger spool valve to increase exhaust gas flow speed towards the turbine;
Antilag system can also help reduce lag;
Turbo lag sometimes misunderstood with engine speeds below boost threshold. When engine speed is below the boost threshold rpm then the 'roll-on' acceleration to build speed and rpm of the vehicle could be considerable, maybe even minimum seconds for a heavy vehicle starting at low vehicle speed in a higher gear. This delay for vehicle speed increase is not defined as a turbo lag, it is because of the improper gear selection for boost demand. Once vehicle reaches the sufficient speed to create required rpm to reach boost threshold, there will be a far shorter delay while the turbo itself builds rotational energy and transitions to positive boost, this last part of the delay in achieving positive boost is referred as the turbo lag;
Turbocharger depends on the flow of exhaust gas through turbine housing. As the engine RPM increases, turbo spools up and starts to build required boost. Delay between opening the throttle and turbo spinning and producing required boost is known as ‘turbo lag’;