GB2297585A

GB2297585A - Charging two-stroke engines

Info

Publication number: GB2297585A
Application number: GB9502044A
Authority: GB
Inventors: Norman David Griffiths
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-02-02
Filing date: 1995-02-02
Publication date: 1996-08-07
Anticipated expiration: 2015-02-02
Also published as: GB9502044D0; GB2297585B

Abstract

Scavenging air from a turbocharger 10, a rotating or reciprocating engine-driven compressor or the engine crankcase (Fig. 3) displaces exhaust gases through an exhaust valve 7 when the volume of the working space is near the maximum. Higher pressure air is supplied through an inlet valve 6 after exhaust valve closure by an engine driven rotary vane compressor 13 or a reciprocating compressor (Fig. 3). The turbocharger 10 or a further turbocharger may supply the compressor 13. The rotary vane compressor (23, Fig. 4) may provide the scavenging and higher pressure air from respective outlets (19, 20, Fig. 2). The supply of higher pressure air may be controlled to provide a substantially constant charge mass over the engine speed range.

Description

SUPERCHARGED TWO-STROKE INTERNAL COMBUSTION ENGINE.

This invention relates to internal combustion engines and is mainly applicable to reciprocating piston engines operating on a two-stroke cycle although certain aspects of it may also be applicable to other types of engine.

It is well established that supercharging an internal combustion piston engine increases the efficiency, power to veight ratio and flexibility, provided the total mass of charge is controllable. In conventional two-stroke engines the mass of charge trapped in the engine cylinder at the start of the compression stroke depends on the back-pressure of the exhaust system: An the engine speed increases, the back-pressure of the exhaust gases also increases and a greater mass of charge is trapped in the engine cylinder.The purpose of this invention is to provide a supercharged internal combustion engine operating on a two-stroke cycle in which the total mass of charge may be controlled and, if desired, kept constant throughout the engine speed range. Furthermore, it is intended to increase the maximum operating speed of two-stroke compression ignition engines which is normally limited by the increase in the mass of charge at higher speeds.

According to this invention there is provided an internal combustion engine including an enclosed working space, a piston movable cyclically in sealed manner to enlarge and contract said working space, a first gump or scavenge pump to supply scavenging air and part of the new charge into said working space at a first pressure while the volume of said working space is near msuui, a second pump or supercharger to supply the rest of the charge into said working space at a second pressure which is higher than said first pressure while the volume of said working space is being reduced but before final compression and combustion, the mass of charge delivered by said second puip being reduced as the speed increases so that the total M's of charge trapped in said working space during final compression and or bastion pey be regulated and kept stable throughout the engine speed range.

As the engine speed increases, a greater mass of scavenging air is trapped in the working space of the engine at the start of each compression stroke. To compensate for that, the mass of charge delivered by the second pump may be reduced to regulate the total mass of charge in the cylinder during combustion.

The first pump fly be a turbocharger driven by exhaust gases, a rotary or reciprocating compressor driven by the engine output shaft and supplied with air to it's intake by a turbocharger, or it may be the engine cr nkease, possibly supplied with air by a turbocharger.

The second pump or supercharger may be a rotary or reciprocating compressor directly or indirectly driven by the engine output shaft.

Various embodiments of the invention are described by way of example, with reference to the accompanying drawings which are essentially diagrammatic in nature and will be well understood by those versed in the art of piston engine design, in whicht Figure 1 shows a schematic layout of the components of an internal combustion engine, partly in section through planes including the axes of various components.

Pigure 2 shows a modified compressor which may be particularly applicable to this invention.

Figures 3 & 4 show alternative layouts for employing the basic principle.

In Figure 1 an internal combustion engine of the two-stroke cycle type includes a cylinder 1, reciprocating piston 2 and cylinder head 3, together enclosing a working space 4 into which air is admitted through an inlet port 5 and an inlet valve 6. Hazel mixed with the air is burnt while the working space 4 is near its minimum volume to drive the piston 2 down the cylinder 1 during the expansion stroke, at the end of which an exhaust valve 7 opens to release products of combustion along a duct 8 to drive a turbine 9 in a turbocharger 10.The turbine 9 drives a compressor or pump 11 which feeds compressed air along a second duct 12 to the inlet port 5 through the cylinder wall 1. liter the exhaust valve 7 has opened, the piston 2 continues to move down the cylinder 1 until it uncovers the port 5, allowing the scavenging air from the turbocharger 10 to enter and purge the working space 4.

As the piston 2 starts to move up the cylinder 1 it first covers the port 5 and then performs a short exhaust stroke, pushing any remaining products of combustion and some of the scavenging air out past the exhaust valve 7. The exhaust valve 7 closes when the piston 2 is part way up the cylinder 1, trapping most of the scavenging air inside the working space 4. As the piston 2 continues to move up the cylinder 1 on the compression stroke, the inlet valve 6 opens to admit a charge of high pressure air into the working space 4 before final compression and combustion occur in the normal manner, after the inlet valve 6 has closed. The high pressure air is delivered to the inlet valve 6 by a high pressure compressor 13.

When and how the fuel is added to the air depends on the type of ignition employed. This type of engine generally employs compression ignition, in which case the fuel would be inJected into the air when it is highly compressed in the working space 4. If spark ignition is employed the fuel would be added to the air before or during it's entry into the working space 4.

At high power outputs it may be possible for the turbocharger 10 to retrieve far more waste energy from the exhaust gases than is required for supplying scavenging air. To make use of any surplus reclaimed energy the turbocharger 10 may also supply pressurised air to the intake of the high pressure compressor 13. It may then be an advantage to employ a valve to regulate the quantity and pressure of the scavenge air delivered into the working space 4 so that air may be fed to the intake of the compressor 13 at a higher pressure. Purthermore, two turbochargers may be employed: A primary turbocharger to supply the scavenging air into the working space 4 and a secondary turbocharger to supply air to the compressor 13.

With compression ignition engines it is an advantage to prevent the mass of the effective charge from varying significantly at different engine speeds. As the speed of this engine increases there will be sn increase in the mass of scavenging air trapped in the working space 4 when the exhaust valve 7 closes, due to the increase in back-pressure of the exhaust gases. To compensate for this the amount of air delivered from the high pressure compressor 13 into the working space 4 each cycle may be reduced as the engine speed increases, keeping the effective charge constant.

The simplest but least efficient method of controlling the delivery rate of air from the compressor 13 is to provide a throttle at it's intake. Alternatively, if the compressor 13 is of a rotary type it may be driven via a viscous coupling or variable slip device. Another method is to tap off excess high pressure air and employ it in a different manner: It may be stored for use during starting and accelerating or it may be used to power air brakes or other pneumatic equipment. If the compressor 13 has a progressive compression process such as in the rotating vane type of compressor, valves may be used to allow some air to escape without being compressed, thereby regulating the amount of high pressure air passed through it's outlet.

Figure 2 shows a rotating vane compressor which includes a hub or barrel 14 with slots carrying vanes 15. The hub 14 is mounted eccentrically in a housing 16 so that there is a minimum amount of clearance between the hub 14 and the housing 16 at one point, but a large clearance diametrically opposite it. The vanes 15 form seals creating egsentsw the volume of which enlarges and contracts as the vanes 15 are carried around by the rotating hub 14. Air entering the inlet port 17 is trapped in the segments, caoressed and pushed out through the outlet port 18. Valves 19 and 20 are provided to allow air to escape from the housing 16 without being compressed or passed through the outlet port 18.

Air fly be supplied to the inlet port 17 by the turbocharger 10 at a pressure substantially above that of the atmosphere. If the valve 19 is open and conanicates with the intake of the turbocharger 10, the pressure in the segment adjacent to the valve 19 will be below that of the atmosphere. The result being that the pressurised air entering the inlet port 17 from the turbocharger 10 will help to drive the hub 14 around, reducing the power input needed to compress the reduced mass of air remaining in the segments. Depending on the position of the second valve 20 it My co-unicate with the first valve 19, the inlet port 17 or the atmosphere.

Figure 3 shows how the basic principle may be applied to a crankcase compression engine. As the piston 2 moves down the cylinder 1 it reduces the volume of the crankcase 21 and increases the pressure of the air in it. When the piston 2 uncovers the port 5 scavenging air from the crankcase 21 flows up the passage 22 and into the working space 4. liter the piston 2 has covered the port 5 the high pressure compressor 13 delivers the rest of the charge through a one-way valve 24 into the working space 4.

Pigure 4 shows how a rotary compressor 23 driven by the engine output shaft nay supply scavenging air and part of the new charge to the working space 4 via the port 5, and high pressure air via the inlet valve 6. The turbocharger 10 supplies air to the intake of the compressor 23 which then acts as a regulator to deliver the correct amount of charge to the port 5 and inlet valve 6.

Claims

CLH

1. An internal combustion engine operating on a two-stroke cycle wherein the new charge of air is delivered into the engine cylinder from two sources and in two stages, the first stage being the supply of scavenging air and part of the new charge while the volume of said cylinder is near maxi, the second stage being the rest of the charge supplied at high pressure when the volume of said cylinder is reduced but before final compression and combustion, the mass of air delivered during said second stage being controlled so that the total mass of charge trapped in said engine cylinder during combustion may be kept stable throughout the engine speed range.

2. An internal combustion engine operating on a two-stroke cycle comprising cylinders with reciprocating pistons and piston controlled ports, cylinder head with valve mechanisms, turbocharger driven by exhaust gases to supply scavenging air and part of the new charge into each cylinder while the volume of said cylinder is near maximum and a high pressure compressor directly or indirectly driven by the engine output shaft to supply further air into each cylinder when the volume of said cylinder is reduced but before final compression and combustion wherein said turbocharger may also supply pressurised air into said high pressure compressor while the delivery rate of air from said high pressure compressor into said cylinders may be controlled to allow the total mass of charge trapped in said cylinders during combustion to be kept stable throughout the engine speed range.

3. An internal combustion engine operating on a two-stroke cycle employing a turbocharger driven by exhaust gases to provide scavenging air and part of the new charge into each cylinder while the volume of said cylinder is near maxinnun and a high pressure compressor to deliver further air into said cylinder when the volume of said cylinder is reduced but before final compression and combustion, said turbocharger also being able to deliver pressurised air into said high pressure compressor at a greater mass flow rate than is required, any excess air providing energy to help drive said high pressure compressor.

4. An internal combustion engine operating on a two-stroke cycle comprising cylinders with reciprocating pistons and piston controlled ports, cylinder head with valve mechanisms, primary turbocharger to supply scavenging air and part of the new charge into said cylinders, high pressure compressor to supply further air into said cylinders and a secondary turbocharger to supply pressurised air to the inlet of said high pressure compressor.

5. An internal combustion engine employing a compressor driven by the engine crankshaft to supply air into the engine cylinders, in which the mass flow rate of air from the compressor into the engine is controlled by allowing excess air to be removed from the compressor before it is compressed to the required delivery pressure.

6. An internal combustion engine substantially as described herein with reference to Pigures 1 to 4 of the accompaaying drawings.