.home page .back Tutorials. IFR
 |
A turbocharged engine is a engine featuring a turbocharger. A turbocharger is a device allowing to provide, at a high altitude, compressed inducted air to a reciprocating engine, and allowing the latter to its best performance. This is because the action through richness, which regulates the fuel-air mixture, eventually comes to its limits. Some GA planes are equipped with a turbocharged engine. The FS2002 Mooney Bravo is of that king
A turbocharger is located between the air intake and the engine as it compresses the air arriving through the plane's air intake before sending it to the engine. The turbocharger's turbine is powered through the exhaust gases. As a turbocharger may produce manifold pressur in excess of the maximum allowable for the plane, a partial waste gate is used so that some of the exhaust may be discarded and only the right amount conserved. The waste gate is automatically regulated by the turbocharger process itself. Thus once the wanted manifold pressure reached, for example, no more action is needed on the gas throttle when a change of altitude occurs. A turbocharger may works according to two main processes. Either a turbocharger maintain maximum sea level maniforld pressure (of the king 29-30 inches Hg) to a certain altitude referred to like the plane's critical altitude, a process called 'altitude turbocharging' or 'normalizing' with the manifold pressure decreasing beyong. Or a turbocharger applies in flight more than the standard 29 inches of manifold pressure, a process called 'ground boosting', with manifold pressures at takeoff to go as high as 45 inches of mercury. Generally speaking, as the air reaching the air manifold is compressed, thus heated some loss of power always exist, whatever the altitude. Some turbocharged systems include a density automatic controller which maintain, through the waste gate, a constant manifold pressure. As far as the waste gate generally is concerned beyond the plane's critical altitude, any speed increase brings to a change in the manifold pressure due to the waste gate being closed in any case and the air reaching the plane's air intake having a relative larger ram pressure, which is transfered to the engine via the turbocharger
A turbocharged engine allows to higher flight altitudes. Like the Mooney Bravo which has a published ceiling of 25,000 ft, at which altitudes he can further maintain a high speed. The critical altitude of the Mooney Bravo is 18,000 ft as it looks like the turbocharger is of the ground boosting type
As far as the use of the turbocharged engine is concerned, any action on the engine controls have to be slow and gentle as engine gauges have to be monitored during any operation. Any overboosting of manifold pressure has to be avoided for a given flight regime
A carefull monitoring of two temperature values had to be performed. The one of the TIT, or turbine inlet temperature -or the EGT or exhaust gas temperature, and of the cylinder head temperature! Which is meant to avoid any deterioration of the turbocharger or the engine. The control which impacts the most the TIT and EGT temperatures is mixture as that control regulates the fuel to air ratio. A temperature excess during the after-takeoff climb does not pose any problem as the full enriched mixture is cooling swiftly. In the case of a climb at cruise regime, one may have to open the cowl flaps and to increase speed. Conversely the turbocharged engine is also more prone to cooling stress, which may be the cas when one throttles back or that the plane descends. The pilot then has to proceded with gradual decrease of gas or to closely monitor temperatures during a descent. He may even have to go to lower the landing gear so to regulate the engine power decrease and allow to a slower cooling. The mixture, generally, must be slightly leaned then, wich eliminates the low power roughness. During a prolonged descent aboard the Mooney Bravo, the pilot should use instead the aerobrakes. He will thus maintain a high r.p.m. to avoid a abrupt engine cooling as keeping a desired speed altogether. In any case aboard a Mooney, one has to avoid any prolonged descent when the manifold pressure is inferior to 15 inches of mercury
A turbocharged engine is controlled through the manifold pressure, tachometer, the TIT, EGT, and cylinder head temperature as settings are applied through the gas throttle, propeller r.p.m., mixture and cowl flaps. During a cruise, gaz should be set at 75 percent or less and the mixture set. Once the gas throttle set, generally, the turbocharger regulates itself on its own. When increasing the airspeed above the critical altitude, the manifold pressure has to be readjusted due to the effect described below. All controls needed for a turbocharged engine are to be found on the FS2002 Mooney Bravo panel. It is of note however that the FS settings are such that the manifold pressure variation relative to r.p.m. is true only in the case when the waste gate should be opened. Which means that when one increases r.p.m., the manifold pressure decreases and conversely as FS did not modelize the case of a closed waste gate, when a r.p.m. increase brings also a manifold pressure increase (and inversely)
Each turbocharger manufacturer usually provides with its own procedures in case of failure. Any trouble with a turbocharger in flight poses important problems due to that high temperatures and pressures are at play
