Deep miller cycle and method for improving thermal efficiency of diesel engine in high compression ratio environment

Abstract

The application discloses a method for improving thermal efficiency of a diesel engine in a deep Miller cycle and a high compression ratio environment, and comprises the following steps: selecting a high compression ratio piston, wherein the geometric compression ratio of the piston is (18-22):1, and delaying an intake valve closing timing (IVC) by more than 60 degrees of a crankshaft rotation angle CA; and selecting a high-flow supercharger, wherein the supercharger stably works under a supercharging ratio greater than 4.7. The application can overcome problems such as insufficient supercharging capacity, unstable working point and limited thermal efficiency improvement when the deep Miller cycle and the high compression ratio piston are applied in the prior art, and maximizes effective thermal efficiency (BTE) of a target narrow-range working condition under the premise that the maximum explosion pressure does not exceed a safety limit.

Description

Technical Field

[0001] This invention relates to the field of engine combustion system technology, and in particular to a method for improving the thermal efficiency of diesel engines in deep Miller cycle and high compression ratio environments. Background Technology

[0002] To meet increasingly stringent emission regulations and energy-saving requirements, improving diesel engine thermal efficiency is a core industry objective. The Miller cycle reduces the effective compression ratio and increases the expansion ratio by delaying intake valve closing (LIVC), thereby reducing pumping losses and improving thermal efficiency. However, in applications with limited knock pressure (e.g., ≤220 bar), simply using the Miller cycle can lead to reduced power output and worsened combustion due to decreased in-cylinder charge.

[0003] Existing technologies typically employ increased geometrical compression ratio (GCR) to compensate for the compression-end temperature and pressure losses caused by the Miller cycle, thereby maintaining good combustion stability. However, this combination of "high compression ratio + deep Miller cycle" places extremely high demands on the turbocharging system: on the one hand, higher intake pressure is required to compensate for the volume loss due to late intake valve closing; on the other hand, traditional turbochargers often operate outside their high-efficiency range under target narrow operating conditions (such as 1400-1800 rpm, 70%-80% load), even approaching the surge boundary, failing to provide stable and efficient turbocharging capabilities. Furthermore, existing solutions lack systematic coupling optimization of the piston combustion chamber structure, valve timing, and turbocharger characteristics, resulting in limited improvement in thermal efficiency.

[0004] The above background information is provided only to aid in understanding the concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was disclosed on the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0005] The primary objective of this invention is to propose a diesel engine thermal efficiency improvement scheme, aiming to overcome the problems of insufficient boosting capacity, unstable operating point, and limited thermal efficiency improvement encountered in the application of deep Miller cycle and high compression ratio pistons in existing technologies. By constructing a three-in-one coupled optimization system of "deep Miller cycle - high compression ratio piston - high flow turbocharger", the effective thermal efficiency (BTE) under the target narrow operating conditions is maximized while ensuring that the maximum burst pressure does not exceed the safety limit.

[0006] Therefore, this invention proposes a method to improve the thermal efficiency of diesel engines in deep Miller cycle and high compression ratio environments.

[0007] Preferably, the present invention may also have the following technical features:

[0008] A method for improving the thermal efficiency of a diesel engine in a deep Miller cycle and high compression ratio environment includes: selecting a high compression ratio piston with a geometric compression ratio of (18-22):1, delaying the intake valve closing timing (IVC) by more than 60° crankshaft angle CA; and selecting a high-flow turbocharger that operates stably under a boost ratio >4.7.

[0009] Furthermore, the intake valve closing timing (IVC) is delayed by more than 80° CA.

[0010] Furthermore, the intake valve closing timing (IVC) is delayed by 80°-100° CA.

[0011] Furthermore, the maximum compressor flow rate of the high-flow-rate booster is ≥0.4518 kg / s.

[0012] Furthermore, the high-flow turbocharger turbine has a corrected flow rate ≥ 0.03675 (kg / s)-K. 0.5 / kPa.

[0013] Furthermore, advance the exhaust valve opening timing (EVO) by 31°CA.

[0014] Furthermore, based on the measured road spectrum data, the core optimized operating conditions were determined to be the torque range of 1400-1800 rpm and 1200-1600 N·m, with 1600 rpm and 70% load as the benchmark.

[0015] Furthermore, the piston geometry compression ratio is 20:1; at the same time, the intake valve closing timing (IVC) is delayed from the original 557.5°CA to 637°CA, and the exhaust valve opening timing (EVO) is advanced from 159°CA to 128°CA.

[0016] Furthermore, it was verified that the high-flow turbocharger, at 1600 rpm and 70% load, with a maximum burst pressure of 220 bar, can stably output an effective thermal efficiency (BTE) of 47.80%.

[0017] Furthermore, system integration and verification are carried out, including bench testing, to verify its performance across the entire operating range and ensure that the design objectives are met.

[0018] The beneficial effects of this invention compared to existing technologies include: by delaying the crankshaft angle, the expansion ratio can be significantly increased, reducing pumping losses. The high compression ratio piston compensates for the drop in compression-end temperature and pressure caused by the deep Miller cycle, allowing for earlier injection timing (SOI) and combustion to more closely resemble isochoric combustion, thereby significantly improving indicated thermal efficiency (ITE). Simulations show that, under ideal conditions, this combination can achieve an ITE of over 53.68%. Attached Figure Description

[0019] Figure 1 This is the control flowchart of the present invention. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to specific embodiments and the accompanying drawings. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope or application of the present invention.

[0021] Non-limiting and non-exclusive embodiments will be described with reference to the following figures, wherein the same reference numerals denote the same parts unless otherwise specifically stated.

[0022] like Figure 1 The method for improving the thermal efficiency of a diesel engine in a deep Miller cycle and high compression ratio environment includes: selecting a high compression ratio piston with a geometric compression ratio of (18-22):1, delaying the intake valve closing timing (IVC) by more than 60° crankshaft angle CA; and selecting a high-flow turbocharger that operates stably under a boost ratio >4.7.

[0023] A high compression ratio piston is selected: a piston with a geometric compression ratio of 20:1 is installed; the intake valve closing timing (IVC) is delayed by more than 60°CA crankshaft angle relative to the original engine; preferably, the intake valve closing timing (IVC) is delayed by more than 80°CA relative to the original engine. By delaying the crankshaft angle, the expansion ratio can be significantly increased, and pumping losses can be reduced. The high compression ratio piston compensates for the drop in compression end temperature and pressure caused by the deep Miller cycle, allowing the injection timing (SOI) to be advanced, and combustion to be closer to isochoric combustion, thereby significantly improving indicated thermal efficiency (ITE). Simulations show that, under ideal conditions, this combination can achieve an ITE of over 53.68%.

[0024] Generally, deep Miller cycles (LIVC ≥ 80°CA) lead to a decrease in effective compression ratio and insufficient in-cylinder compression-end temperature / pressure, causing combustion delay and a later peak heat release rate, thus reducing thermal efficiency. However, this invention directly compensates for the loss of effective compression ratio caused by late intake valve closing by increasing the piston geometric compression ratio (GCR) to 20:1. For example, when LIVC = 80°CA, the effective compression ratio drops to approximately 13.5:1; while the GCR = 20:1 design maintains the compression-end pressure (Pc) at ≥ 45 bar and the temperature (Tc) at ≥ 850 K, ensuring that the fuel ignition delay period remains stable within the ideal range of 2-3 ms. This allows the combustion phase (CA50) to be advanced to 8-10°CA after top dead center, significantly improving indicated thermal efficiency (ITE) and laying the foundation for subsequent BTE improvements.

[0025] The combination of GCR=20:1 + LIVC=80°CA improves combustion, but leads to an increase in the residual exhaust gas coefficient in the cylinder and a surge in the demand for fresh charge density.

[0026] A high-flow-rate turbocharger is selected: the turbocharger's compressor has a maximum flow rate of 0.4518 kg / s, a maximum efficiency of 80.18%, and can operate stably at high pressure ratios (>4.7). The turbocharger turbine has a corrected flow rate of 0.03675 (kg / s)-K. 0.5 / kPa, its flow characteristics have been optimized to meet the strong boost demand under medium and high loads, while also taking into account low load performance to a certain extent. By selecting a compressor and turbine that meet the requirements, the operating point is located in the center of the high-efficiency region of the compressor map and far away from the surge boundary under the target optimized operating conditions (1600 rpm, 70% load), ensuring the stability and efficiency of the boost process.

[0027] Turbine correction flow rate (0.03675 (kg / s) - K) 0.5 The turbocharger ( / kPa) ensures that the compressor can reach the required speed (>180,000 rpm) in the medium-load region where exhaust energy is limited, avoiding boost lag. The stable high-flow intake provided by the turbocharger directly solves the problems of insufficient charge and combustion oxygen deficiency that high-compression-ratio pistons may face under deep Miller cycles, enabling the system to operate safely within the 220 bar explosion pressure limit. The effective thermal efficiency (BTE) reaches 47.2%.

[0028] With the support of the aforementioned strong boost, the system's expansion work potential is fully released, but late exhaust opening will lead to an increase in pumping negative work, offsetting part of the thermal efficiency gain.

[0029] Therefore, in some embodiments, the exhaust valve opening timing (EVO) is advanced by 31°CA relative to the original engine. Although opening the exhaust valve earlier results in a loss of some expansion work, under the high expansion ratio and high boost configuration of this invention, the turbocharger has already established a sufficiently high intake pressure (>4.5 bar) before the exhaust valve opens, and residual exhaust gas in the cylinder is efficiently removed, significantly reducing pumping losses (measured reduction of approximately 8%). Opening the exhaust valve earlier can more effectively reduce pumping negative work, ultimately achieving a net increase in effective thermal efficiency (BTE). In this embodiment, under the premise of constant explosion pressure, this operation results in a net increase in effective work output, ultimately further increasing the BTE from 47.2% to 47.8%.

[0030] Through multi-stage coupling enhancement, under the baseline operating conditions of 1600 rpm and 70% load, the effective thermal efficiency (BTE) increased from 45.29% of the original unit to 47.80%, an absolute increase of 2.51 percentage points, resulting in a significant improvement in thermal efficiency.

[0031] Moreover, BTE improved by more than 2.3 percentage points across the entire target narrow range (1400-1800 rpm, medium load), achieving comprehensive coverage of typical tractor operating conditions and optimizing wide-range performance.

[0032] At the same time, with the cooperation of the high-flow turbocharger, the problem of the original turbocharger operating point being biased to the left and prone to surge under deep Miller cycle was completely solved, ensuring the reliability of engine operation.

[0033] This invention was completed under the safety constraint of a maximum burst pressure ≤220 bar, ensuring the structural safety of the engine and meeting the burst pressure limit.

[0034] Take an inline six-cylinder hybrid diesel engine as an example.

[0035] First, based on the measured road spectrum data, the core optimized operating conditions were determined to be the torque range of 1400-1800 rpm and 1200-1600 N·m, with 1600 rpm and 70% load as the benchmark.

[0036] Step 1: Piston and valve train modification. Replace the piston to achieve a geometric compression ratio of 20:1. Simultaneously, preferably use a camshaft adapted to the high compression ratio piston, retarding the intake valve closing timing (IVC) from the original 557.5°CA to 637°CA (i.e., LIVC80), and advancing the exhaust valve opening timing (EVO) from 159°CA to 128°CA.

[0037] Step 2: Turbocharger matching. Remove the original turbocharger and install a high-flow-rate new turbocharger. Verification was performed using a one-dimensional simulation model built with GT-Power. At 1600 rpm and 70% load, with the maximum burst pressure set at 220 bar, the system could stably output 47.80% of BTE.

[0038] Step 3: System Integration and Verification. Integrate the optimized piston, camshaft, and high-flow intensifier into the complete machine and conduct bench tests to verify its performance across the entire operating range, ensuring that the design objectives are met.

[0039] Those skilled in the art will recognize that numerous variations are possible with respect to the above description, and the embodiments and figures are merely for describing one or more specific implementations.

[0040] Although exemplary embodiments of the invention have been described and illustrated, those skilled in the art will understand that various changes and substitutions can be made thereto without departing from the spirit of the invention. Furthermore, many modifications can be made to adapt specific situations to the doctrine of the invention without departing from the central concepts of the invention described herein. Therefore, the invention is not limited to the specific embodiments disclosed herein, but may include all embodiments and equivalents that fall within the scope of the invention.

Claims

1. A method for improving the thermal efficiency of a diesel engine in a deep Miller cycle and high compression ratio environment, characterized in that: include: A high compression ratio piston is selected, with a geometric compression ratio of (18-22):1, and the intake valve closing timing (IVC) is delayed by more than 60° crankshaft angle CA; a high flow rate turbocharger is selected, which operates stably under a boost ratio >4.

7.

2. The method as described in claim 1, characterized in that: Intake valve closing timing (IVC) is delayed by more than 80° CA.

3. The method as described in claim 2, characterized in that: Intake valve closing timing (IVC) is delayed by 80°-100° CA.

4. The method as described in claim 1, characterized in that: The maximum compressor flow rate of the high-flow-rate booster is ≥0.4518 kg / s.

5. The method as described in claim 4, characterized in that: The high-flow turbocharger turbine has a corrected flow rate ≥ 0.03675 (kg / s) - K. 0.5 / kPa.

6. The method as described in claim 1, characterized in that: Advance the exhaust valve opening timing (EVO) by 31°CA.

7. The method as described in claim 6, characterized in that: Based on the measured road spectrum data, the core optimized operating conditions were determined to be the torque range of 1400-1800 rpm and 1200-1600 N·m, with 1600 rpm and 70% load as the benchmark.

8. The method as described in claim 7, characterized in that: The piston geometry compression ratio is 20:1; the intake valve closing timing (IVC) is delayed from the original 557.5°CA to 637°CA, and the exhaust valve opening timing (EVO) is advanced from 159°CA to 128°CA.

9. The method as described in claim 8, characterized in that: The high-flow booster was verified to achieve a stable output of 47.80% effective thermal efficiency (BTE) at 1600 rpm and 70% load, with the maximum burst pressure set at 220 bar.

10. The method as described in claim 9, characterized in that: System integration and verification include bench testing to verify its performance across the entire operating range and ensure that design objectives are met.

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