1. Field of the Invention
The present invention relates generally to turbocharged engines and, more particularly, to the improved operation of turbocharged engines at low Revolutions-Per-Minute (RPM).
2. Description of the Prior Art
Diesel engines may be either 2-Stroke or 4-Stroke cycle. Diesel engines differ from gasoline engines in that ignition occurs as a result of the heat from near-adiabatic compression. As historically-implemented, the naturally-aspirated diesel is a high compression ratio engine (typically between 16 and 22 to 1). As such, the compression ratio is approximately twice the compression ratio of a gasoline engine. Diesel engines therefore have lower power to weight ratio than gasoline engines. Modern high speed diesel engines have largely compensated for the inherent weight penalty of the diesel engine by the use of turbo-charging and after-cooling.
In the turbocharged and after-cooled marine diesel, the waste heat in the exhaust (typically about 1000 degrees Fahrenheit at full load) is used to spin a turbine, which drives a compressor which consequently boosts the intake manifold pressure. The combination of the pressurized intake manifold and the aftercooler (or intercooler), which cools the air downstream of the compressor, increases the power of the turbocharged diesel by a factor of approximately two or three. This is comparable to the power to weight ratio of gasoline engines.
With a turbocharged engine, the power at low RPMs may be one third or less of the power at higher RPMs. For example, a typical high speed diesel may be rated at 370 horsepower at 2400 RPMs; the diesel rates being approximately 75 horsepower at 1000 RPM. This low horse power condition is because at low RPMs there is not sufficient exhaust flow to spin the turbocharger fast enough to create a boost in horsepower.
In many applications, this difference in horsepower at different RPMs is not important because transmissions or gear boxes may be utilized to adjust the load to allow fast acceleration of the engine to higher speeds. With boats, various means have been utilized to minimize or avoid this problem including the use of variable pitch propellers, gearboxes, engine-powered superchargers, and operationally-comparable mechanisms. Larger engines may also be utilized. However, these solutions add considerable costs.
When planning craft of 20-50 feet in length, operating at top speeds of 25-50+ knots, the most efficient propulsor at design point (top speed) will generally be a fixed pitch propeller. With the fixed pitch propeller (surface piercing or not), unless slip is significant, speed tends to be more or less linear with the RPM. As a result, the engine tends to be at a low RPM when the vessel is required to “climb out of the hole “onto” plane.
If the vessel has a low-to-moderate top speed compared to the hull speed (2-2.5 times hull speed or less); generally there is no problem. The engine will be above the critical one-half rated speed point and will have the ability to develop boost and power before that power is needed to transition to a plane condition. However, as designs for faster speeds are made (3-5 times hull speed), a situation can exist where if the vessel is perfectly “propped’ for the full speed condition there will not be significant (or in some cases any) excess power and the vessel will either be slow to get onto plane, or may be unable to transition to a plane condition.
As an example, a 33 foot craft with a 22,000 lb. displacement powered by two 370 hp diesels, a top speed in excess of 32 knots can be achieved at the proper RPM (2825). When fuel and water loads are increased to full (an additional 1500 lb. displacement) the vessel cannot transition onto plane in shallow water, and in deeper water requires 20-30 seconds at full throttle from a standing start before any boost pressure is obtained. During this 20-30 second period of time, the vessel runs at 9-10 knots and smokes. Once the boost starts, the vessel breaks onto plane and accelerates to top speed in less than ten seconds. The compromise solution is to reduce the propeller diameter by 1″ from 22″×24″ to 21″×24″. This solution sacrifices 3 knots of top speed (new speed 29 knots) but allows sufficient slip during acceleration that boost is achieved in 8 seconds or less. A ten percent pitch reduction at the same diameter (considered) would have had the same effect.
Neither solution is acceptable for naval applications. Navy craft need good performance/efficiency over a much wider range of load conditions than is typical of most vessels. In addition, smoke is created during this temporary overload (fuel rich condition). Engines which have electronic controls or other fuel/smoke limiters have an even greater problem with this transition.
The above-discussed solutions such as gearboxes, larger engines, and the like might be utilized, but the cost is high. The following United States Patents describe various prior art systems that may be related to the above and/or other turbocharger systems:
U.S. Pat. No. 4,392,352 (issued Jul. 12, 1983) to Stumpp et al, discloses a method and an apparatus for regulating and attaining an anti-overload means in turbochargers and in internal combustion engines equipped therewith, serving in cost-favorable embodiment both in Otto (gasoline) engines and in diesel engines to limit the air throughput of the engine in accordance with the engine speed and to provide that neither the turbocharger (as a result of exceeding its limit rpm) nor the engine can be endangered if the engine exceeds the permissible compression and combustion pressure.
In summary, the intent is to attain a favorable adjustment of the exhaust turbocharger over the entire RPM range of the internal combustion engine. To this end, the exhaust gas quantity delivered to the exhaust turbocharger is controlled in accordance with the throughput, that is, in accordance with the air quantity delivered to the engine, with the aid of a bypass line. The air quantity delivered by way of the compression area of the exhaust turbocharger of the engine is detected ahead of the exhaust turbocharger, either with the aid of a direct air flow rate meter, or by means of a throttle restrictor and the detection of the underpressure being created at that point.
U.S. Pat. No. 5,131,229 (issued Jul. 21, 1992) to Kriegler, discloses that in order to utilize recycling of exhaust gases at high engine loads in an internal-combustion engine with an exhaust gas turbocharger, optionally with a charge cooler, as well as an exhaust gas recycling valve which is arranged within a connecting pipe through which a partial exhaust gas stream flows, the connecting pipe, in the direction of the flow, branching from the exhaust pipe upstream of the exhaust gas turbine and connecting into the charge pipe downstream of the exhaust gas turbine, an apparatus is employed which injects water into the partial exhaust gas stream flowing the connecting pipe at operating temperature and at high load operation of the internal-combustion engine.
U.S. Pat. No. 6,422,008 (issued Jul. 23, 2002) to Voss et al, discloses methods and apparatus for reducing the RPM level of a diesel engine exhaust stream by providing a suitable oxidation catalyst into the exhaust train. The oxidation catalyst may be incorporated into a thermal insulative coating on the inner surface of the exhaust train-particularly the exhaust manifold and exhaust pipes prior to the turbocharger. Alternatively, when the exhaust train includes a turbocharger, the catalyst can be in a separate monolithic unit between the engine and the turbocharger. The system may also include an improved diesel oxidation catalyst unit having a metal monolithic substrate. The oxidation catalyst can also be incorporated into a thermal insulative coating inside the cylinders, particularly on non-rubbing surfaces. A further embodiment is the use of a stainless steel bond coat to bind the thermal coating to a metallic substrate, particularly an aluminum substrate.
U.S. Pat. No. 6,470,866 (issued Oct. 29, 2002) to Cook, discloses an apparatus for and method of exhaust gas recirculation in an internal combustion engine that operates with charge air boost. An EGR valve has an inlet port communicated to the engine exhaust system upstream of a throttle valve in the tailpipe and an outlet port communicated to the engine intake system. The throttle valve is controlled to selectively restrict exhaust gas flow through the tailpipe so as to maintain the difference between pressure at the EGR valve inlet and pressure at the EGR valve outlet substantially unaffected by changes in pressure in the intake system and in the exhaust system.
U.S. Pat. No. 6,557,347 (issued May 6, 2003) to Alvarez et al, discloses a method and apparatus of operating a turbo-charged diesel locomotive engine to facilitate controlling pressure in an engine cylinder. The method includes determining an allowable peak firing pressure for the turbo-charged diesel engine; determining an actual peak firing pressure; and comparing the allowable peak firing pressure to actual peak firing pressure to control the operation of the turbocharger for controlling peak firing pressure. The apparatus includes a diesel engine including an intake manifold, an exhaust manifold, an electronic fuel controller, a turbo-charger, and a motor-generator coupled to the turbocharger and operable to at least one of increased turbocharger rotational speed, decrease turbocharger rotational speed, and maintain turbocharger rotational speed, and a controller including a first input corresponding intake manifold air pressure and a second input corresponding to fuel injection timing for the engine and including as an output a motor-generator configuration signal.
U.S. Pat. No. 6,637,382 (issued October 28) 2003, to Brehob et al, discloses a turbocharger system for a diesel engine includes an exhaust driven intake air compressor, a sensor for tracking the position of the engine's throttle and for generating a throttle position signal, and a water injection system for furnishing water to the engine's air intake. A controller receives the throttle position signal and operates the water injection system such that the rate of water injection will be increased in the event that the time rate of change of the throttle position signal indicates that the throttle pedal is being depressed at a rate exceeding a predetermined threshold.
U.S. Pat. No. 6,955,162 (issued Oct. 18, 2005) to Larson et al, discloses an exhaust gas recirculating system for a turbocharged diesel engine utilizes an electrically driven compression pump to boost exhaust gas pressure before return to the engine induction system. Exhaust gas is drawn from the exhaust system or stack some distance removed and downstream from the outlet from the exhaust turbine, compressed to overcome the intake manifold boost pressure, and returned to the intake system along an extended pipe to cool the gas. The compressor is energized from the vehicle battery during periods of demand for ak pressure demand on the engine thereby recycling recaptured energy from the battery to boost engine output. Exhaust turbine performance during periods of peak loading is also improved.
The above-cited prior art does not adequately disclose a low cost way to improve the operation of a turbocharger powered internal combustion engine at low RPMs. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.